PHP 5 允行开发者在一个类中定义一个方法作为构造函数。具有构造函数的类会在每次创建新对象时先调用此方法，所以非常适合在使用对象之前做一些初始化工作。
Note: 如果子类中定义了构造函数则不会隐式调用其父类的构造函数。要执行父类的构造函数，需要在子类的构造函数中调用 parent::__construct()。如果子类没有定义构造函数则会如同一个普通的类方法一样从父类继承（假如没有被定义为 private 的话）。
Example #1 使用新标准的构造函数
<?php
class BaseClass {
function __construct() {
print "In BaseClass constructor\n";
}
}
class SubClass extends BaseClass {
function __construct() {
parent::__construct();
print "In SubClass constructor\n";
}
}
class OtherSubClass extends BaseClass {
// inherits BaseClass's constructor
}
// In BaseClass constructor
$obj = new BaseClass();
// In BaseClass constructor
// In SubClass constructor
$obj = new SubClass();
// In BaseClass constructor
$obj = new OtherSubClass();
?>
为了实现向后兼容性，如果 PHP 5 在类中找不到 __construct() 函数并且也没有从父类继承一个的话，它就会尝试寻找旧式的构造函数，也就是和类同名的函数。因此唯一会产生兼容性问题的情况是：类中已有一个名为 __construct() 的方法却被用于其它用途时。
与其它方法不同，当 __construct()
被与父类 __construct()
具有不同参数的方法覆盖时，PHP 不会产生一个 E_STRICT 错误信息。
自 PHP 5.3.3 起，在命名空间中，与类名同名的方法不再作为构造函数。这一改变不影响不在命名空间中的类。
Example #2 Constructors in namespaced classes
<?php
namespace Foo;
class Bar {
public function Bar() {
// treated as constructor in PHP 5.3.0-5.3.2
// treated as regular method as of PHP 5.3.3
}
}
?>
PHP 5 引入了析构函数的概念，这类似于其它面向对象的语言，如 C++。析构函数会在到某个对象的所有引用都被删除或者当对象被显式销毁时执行。
Example #3 析构函数示例
<?php
class MyDestructableClass {
function __construct() {
print "In constructor\n";
$this->name = "MyDestructableClass";
}
function __destruct() {
print "Destroying " . $this->name . "\n";
}
}
$obj = new MyDestructableClass();
?>
和构造函数一样，父类的析构函数不会被引擎暗中调用。要执行父类的析构函数，必须在子类的析构函数体中显式调用 parent::__destruct()。此外也和构造函数一样，子类如果自己没有定义析构函数则会继承父类的。
析构函数即使在使用 exit() 终止脚本运行时也会被调用。在析构函数中调用 exit() 将会中止其余关闭操作的运行。
Note:
析构函数在脚本关闭时调用，此时所有的 HTTP 头信息已经发出。脚本关闭时的工作目录有可能和在 SAPI（如 apache）中时不同。
Note:
试图在析构函数（在脚本终止时被调用）中抛出一个异常会导致致命错误。
To better understand the __destrust method:
class A {
protected $id;
public function __construct($id)
{
$this->id = $id;
echo "construct {$this->id}\n";
}
public function __destruct()
{
echo "destruct {$this->id}\n";
}
}
$a = new A(1);
echo "-------------\n";
$aa = new A(2);
echo "=============\n";
The output content:
construct 1
-------------
construct 2
=============
destruct 2
destruct 1
for multiple constructor like, you can use the constructor this way
class Foo
{
public function __construct(...$args)
{
if (count($args) == 1) {
echo 'One parameter';
} else if (count($args == 2)) {
echo 'Two parameter';
} else {
echo 'More than one parameter';
}
}
}
new Foo('hello', 'world');
The __destruct magic method must be public.
public function __destruct()
{
;
}
The method will automatically be called externally to the instance. Declaring __destruct as protected or private will result in a warning and the magic method will not be called.
Note: In PHP 5.3.10 i saw strange side effects while some Destructors were declared as protected.
This two documents shows how OOP can work with using a constructor.
[Document user.php]
<?php
//Attribute
class user{
private $name; //$n
private $forename; //$fn
private $birthday; //$bd
private $username;
//constructor
public function __construct($n,$fn,$bd)
{
if(strlen($n)>=3 && strlen($fn)>=2) //min 3 chars for name and 2 chars for the forename
{
$this->name=$n;
$this->forename=$fn;
}
$this->birthday=$bd;
}
//Getter-Methode
public function getName()
{
if($this->name!="")
{
return $this->name;
}
else
{
return "<strong style='color:red'> Name required.</strong>";
}
}
public function getForename()
{
if($this->forename!="")
{
return $this->forename;
}
else
{
return "<strong style='color:red'>Forename required.</strong>";
}
}
public function getBirthday()
{
if($this->birthday!="")
{
return $this->birthday;
}
else
{
return "<strong style='color:red'>Birthday required.</strong>";
}
}
public function getUsername()
{
if($this->username!="")
{
return $this->username;
}
elseif($this->name!=""&&$this->forename!="")
{
$this->createUN();
return $this->username;
}
else
{
return"<strong style='color:red'>Username can not created. </strong>";
}
}
//Creating the Username with name and forename.
public function createUN()
{
$username="";
for($i=0;$i<=3;$i++)
{
$username[$i]= $this->name[$i];
}
for($i=0;$i<=1;$i++)
{
$username[$i+2]= $this->forename[$i];
}
$username=implode("",$username); //changes array to string
return $username;
}
}
?>
//-------------------------------------------------------------------------------
[Document user_test.php]
<?php
include("user.php");
$user1= new user("Musterman","Max","31.01.1998"); //Object with contructor
echo $user1->getForename();
echo $user1->getName();
echo $user1->getBirthday();
echo $user1->createUN();
?>
[quote]
Note:
Attempting to throw an exception from a destructor (called in the time of script termination) causes a fatal error.
[/quote]
Not true.
Usually you wont be able to catch an Exception thrown out of a ::__destruct().
BUT following case shows that it IS possible to catch a thrown Exception from the ::__destruct()
<?php
class Something
{
public function __construct()
{
echo __METHOD__ . PHP_EOL;
}
public function __destruct()
{
echo __METHOD__ . PHP_EOL;
throw new Exception("Exception thrown out of ::__destruct()", 1);
}
}
try{
$Something = new Something();
$Something = null; // will cause to call the objects ::__destruct()
// also possible: unset($Something);
}catch(Exception $e){
echo 'Exception: ' . $e->getMessage() . PHP_EOL;
}
echo 'End of script -- no Fatal Error.';
?>
IMO this is not a bug. It is ok to be able to throw and catch Exception.
putting in more layman's term
class dad{
function __construct() {
echo get_class($this) . ' is fair and tall <br>';
}
}
//inherit and overidding parent constructor
class son extends dad{
function __construct(){
parent::__construct();
echo get_class($this) . ' is handsome <br>';
}
}
//overidding parent constructor
class daughter extends dad{
function __construct(){
echo get_class($this) . ' is tan like her mother <br>';
echo get_class($this) . ' is pretty and has nothing to do with dad <br>';
}
}
echo 'im dad: <br>';
$dad = new dad();
echo '<br>';
echo 'born a son: <br>';
$son = new son();
echo '<br>';
echo 'born a daughter: <br>';
$daughter = new daughter();
/*output
im dad:
dad is fair and tall
born a son:
son is fair and tall (notice that when parent:: is call inside son, son will be the class inside dad's contructor)
son is handsome
born a daughter:
daughter is tan like her mother
daughter is pretty and has nothing to do with dad
*/
By inheriting from this class, you can do constructor overloading in terms of type hinting.
<?php
abstract class OverloadedConstructors {
public final function __construct() {
$self = new ReflectionClass($this);
$constructors = array_filter($self->getMethods(ReflectionMethod::IS_PUBLIC), function(ReflectionMethod $m) {
return substr($m->name, 0, 11) === '__construct';
});
if(sizeof($constructors) === 0)
trigger_error('The class ' . get_called_class() . ' does not provide a valid constructor.', E_USER_ERROR);
$number = func_num_args();
$arguments = func_get_args();
foreach($constructors as $constructor) {
if(($number >= $constructor->getNumberOfRequiredParameters()) &&
($number <= $constructor->getNumberOfParameters())) {
$parameters = $constructor->getParameters();
reset($parameters);
foreach($arguments as $arg) {
$parameter = current($parameters);
if($parameter->isArray()) {
if(!is_array($arg))
continue 2;
}elseif(($expectedClass = $parameter->getClass()) !== null) {
if(!(is_object($arg) && $expectedClass->isInstance($arg)))
continue 2;
}
next($parameters);
}
$constructor->invokeArgs($this, $arguments);
return;
}
}
trigger_error('The required constructor for the class ' . get_called_class() . ' did not exist.', E_USER_ERROR);
}
}
?>
Simply define a class like this:
<?php
class Test extends OverloadedConstructors {
public function __construct1(array $arg) {
die('First construct');
}
public function __construct2(stdClass $test) {
die('Second construct');
}
public function __construct3($optional = null) {
die('Third construct');
}
}
?>
You can define as much constructors as you wish and limit them by php's type hinting. Note that the constructor signatures should be non-ambiguous.
If your code has a redundant constructor for PHP 4 compatibility, an E_STRICT error may be raised in PHP 5.x:
Strict Standards: Redefining already defined constructor for class...
What is a redundant constructor for PHP 4 compatibility? It looks like
<?php
Class Foo {
function Foo() {
return $this->__construct();
}
function __construct() {
// whatever
}
?>
If you don't need PHP 4, just delete the Foo function. Or you reverse the order of Foo and __construct, the error goes away, so that's probably OK for a single class. Also there was a suggestion to do this in Foo:
<?php
function Foo() {
if (version_compare(PHP_VERSION,"5.0.0","<")) {
return $this->__construct();
}
}
?>
<?php
/**
* a funny example Mobile class
*
* @author Yousef Ismaeil Cliprz[At]gmail[Dot]com
*/
class Mobile {
/**
* Some device properties
*
* @var string
* @access public
*/
public $deviceName,$deviceVersion,$deviceColor;
/**
* Set some values for Mobile::properties
*
* @param string device name
* @param string device version
* @param string device color
*/
public function __construct ($name,$version,$color) {
$this->deviceName = $name;
$this->deviceVersion = $version;
$this->deviceColor = $color;
echo "The ".__CLASS__." class is stratup.<br /><br />";
}
/**
* Some Output
*
* @access public
*/
public function printOut () {
echo 'I have a '.$this->deviceName
.' version '.$this->deviceVersion
.' my device color is : '.$this->deviceColor;
}
/**
* Umm only for example we will remove Mobile::$deviceName Hum not unset only to check how __destruct working
*
* @access public
*/
public function __destruct () {
$this->deviceName = 'Removed';
echo '<br /><br />Dumpping Mobile::deviceName to make sure its removed, Olay :';
var_dump($this->deviceName);
echo "<br />The ".__CLASS__." class is shutdown.";
}
}
// Oh ya instance
$mob = new Mobile('iPhone','5','Black');
// print output
$mob->printOut();
?>
The Mobile class is stratup.
I have a iPhone version 5 my device color is : Black
Dumpping Mobile::deviceName to make sure its removed, Olay :
string 'Removed' (length=7)
The Mobile class is shutdown.
As of PHP 5.3.10 destructors are not run on shutdown caused by fatal errors.
For example:
<?php
class Logger
{
protected $rows = array();
public function __destruct()
{
$this->save();
}
public function log($row)
{
$this->rows[] = $row;
}
public function save()
{
echo '<ul>';
foreach ($this->rows as $row)
{
echo '<li>', $row, '</li>';
}
echo '</ul>';
}
}
$logger = new Logger;
$logger->log('Before');
$nonset->foo();
$logger->log('After');
?>
Without the $nonset->foo(); line, Before and After will both be printed, but with the line neither will be printed.
One can however register the destructor or another method as a shutdown function:
<?php
class Logger
{
protected $rows = array();
public function __construct()
{
register_shutdown_function(array($this, '__destruct'));
}
public function __destruct()
{
$this->save();
}
public function log($row)
{
$this->rows[] = $row;
}
public function save()
{
echo '<ul>';
foreach ($this->rows as $row)
{
echo '<li>', $row, '</li>';
}
echo '</ul>';
}
}
$logger = new Logger;
$logger->log('Before');
$nonset->foo();
$logger->log('After');
?>
Now Before will be printed, but not After, so you can see that a shutdown occurred after Before.
With the destructor if you're going to write to a file make sure you have the full file path and not just the filename, it seems PHP switches the working directory to the apache root directory when exiting the script.
Be aware of potential memory leaks caused by circular references within objects. The PHP manual states "[t]he destructor method will be called as soon as all references to a particular object are removed" and this is precisely true: if two objects reference each other (or even if one object has a field that points to itself as in $this->foo = $this) then this reference will prevent the destructor being called even when there are no other references to the object at all. The programmer can no longer access the objects, but they still stay in memory.
Consider the following example:
<?php
header("Content-type: text/plain");
class Foo {
/**
* An indentifier
* @var string
*/
private $name;
/**
* A reference to another Foo object
* @var Foo
*/
private $link;
public function __construct($name) {
$this->name = $name;
}
public function setLink(Foo $link){
$this->link = $link;
}
public function __destruct() {
echo 'Destroying: ', $this->name, PHP_EOL;
}
}
// create two Foo objects:
$foo = new Foo('Foo 1');
$bar = new Foo('Foo 2');
// make them point to each other
$foo->setLink($bar);
$bar->setLink($foo);
// destroy the global references to them
$foo = null;
$bar = null;
// we now have no way to access Foo 1 or Foo 2, so they OUGHT to be __destruct()ed
// but they are not, so we get a memory leak as they are still in memory.
//
// Uncomment the next line to see the difference when explicitly calling the GC:
// gc_collect_cycles();
//
// see also: http://www.php.net/manual/en/features.gc.php
//
// create two more Foo objects, but DO NOT set their internal Foo references
// so nothing except the vars $foo and $bar point to them:
$foo = new Foo('Foo 3');
$bar = new Foo('Foo 4');
// destroy the global references to them
$foo = null;
$bar = null;
// we now have no way to access Foo 3 or Foo 4 and as there are no more references
// to them anywhere, their __destruct() methods are automatically called here,
// BEFORE the next line is executed:
echo 'End of script', PHP_EOL;
?>
This will output:
Destroying: Foo 3
Destroying: Foo 4
End of script
Destroying: Foo 1
Destroying: Foo 2
But if we uncomment the gc_collect_cycles(); function call in the middle of the script, we get:
Destroying: Foo 2
Destroying: Foo 1
Destroying: Foo 3
Destroying: Foo 4
End of script
As may be desired.
NOTE: calling gc_collect_cycles() does have a speed overhead, so only use it if you feel you need to.
Php 5.3.3 had a very strange bug, related to __destruct, but this bug was fixed in 5.3.6. This bug happened to me and it took me forever to figure it out, so I wanted to share it.
In short, don't unset properties in __destruct():
<?php
class Foobar {
public $baz;
function __destruct() {
# Don't do either of these, if $baz also has a __destruct()!
$this->baz = null;
unset($this->baz);
# Instead, don't clear it at all, or do this:
$this->baz->__destruct();
}
}
?>
If you made this mistake, this might happen in php<5.3.6:
<?php
# Some function that throws an exception
function fail($foobar) {
throw new Exception("Exception A!");
}
$foobar = new Foobar();
$foobar->baz = new Foobar();
try {
fail($foobar); // Send foobar to func that throws an Exception
} catch( Exception $e ) {
print $e->getMessage(); // Exception A will be caught and printed, as expected.
}
$foobar = null; // clearing foobar, and its property $baz
try {
print 'Exception B:';// this will be printed
// output stops here.
throw new Exception("Exception B!");
} catch( Exception $e ) {
print $e->getMessage(); // doesn't happen
}
print 'End'; // this won't be printed
?>
multiple constructors with type signetures (almost overloading):
<?php
public function __construct() {
$args = func_get_args();
$argsStr = '';
foreach ($args as $arg) {
$argsStr .= '_' . gettype($arg);
}
if (method_exists($this, $constructor = '__construct' . $argsStr))
call_user_func_array(array($this, $constructor), $args);
else
throw new Exception('NO CONSTRUCTOR: ' . get_class() . $constructor, NULL, NULL);
}
public function __construct_integer($a) {}
public function __construct_string($a) {}
public function __construct_integer_string($a, $b) {}
public function __construct_string_integer($a, $b) {}
?>
the easiest way to use and understand multiple constructors:
<?php
class A
{
function __construct()
{
$a = func_get_args();
$i = func_num_args();
if (method_exists($this,$f='__construct'.$i)) {
call_user_func_array(array($this,$f),$a);
}
}
function __construct1($a1)
{
echo('__construct with 1 param called: '.$a1.PHP_EOL);
}
function __construct2($a1,$a2)
{
echo('__construct with 2 params called: '.$a1.','.$a2.PHP_EOL);
}
function __construct3($a1,$a2,$a3)
{
echo('__construct with 3 params called: '.$a1.','.$a2.','.$a3.PHP_EOL);
}
}
$o = new A('sheep');
$o = new A('sheep','cat');
$o = new A('sheep','cat','dog');
// results:
// __construct with 1 param called: sheep
// __construct with 2 params called: sheep,cat
// __construct with 3 params called: sheep,cat,dog
?>
Another way to emulate multiple constructors:
<?php
class MyClass {
// use a unique id for each derived constructor,
// and use a null reference to an array,
// for optional parameters
function __construct($id="", $args=null) {
// parent constructor called first ALWAYS
parent::__construct();
echo "main constructor\n";
// avoid null or other types
$id = (string) $id;
// allow default constructor
if ($id == "") {
$id = "default";
}
// just in case, use lowercase id
$id = "__cons_" . strtolower($id);
$rc = new ReflectionClass("MyClass");
if (!$rc->hasMethod($id) {
echo "constructor $id not defined\n";
}
else {
// using "method references"
$this->$id($args);
}
$rc = null;
} // function __construct
// ALWAYS HAVE A default constructor,
// that ignores arguments
function __cons_default($args=null) {
echo "Default constructor\n";
}
// may have other constructors that expect different
// argument types or argument count
function __cons_min($args=null) {
if (! is_array($args) || count($args) != 2) {
echo "Error: Not enough parameters (Constructor min(int $a, int $b)) !!!\n";
}
else {
$a = (int) $args[0];
$b = (int) $args[1];
$c = min($a,$b);
echo "Constructor min(): Result = $c\n";
}
}
// may use string index (associative array),
// instead of integer index (standard array)
function __cons_fullname($args=null) {
if (! is_array($args) || count($args) < 2) {
echo "Error: Not enough parameters (Constructor fullname(string $firstname, string $lastname)) !!!\n";
}
else {
$a = (string) $args["firstname"];
$b = (string) $args["lastname"];
$c = $a . " " . $b;
echo "Constructor fullname(): Result = $c\n";
}
}
} // class
// --> these two lines are the equivalent
$obj1 = new TestClass();
$obj2 = new TestClass("default");
// --> these two are specialized
// should be read as "$obj3 = new TestClass, min(99.7, 99.83);"
$obj3 = new TestClass("min", array(99.7, 99.83));
// should be read as "$obj4 = new TestClass, fullname("John", "Doe");"
$obj4 = new TestClass("fullname", array("firstname" => "John", "lastname" => "Doe"));
// --> these two lack parameters
$obj5 = new TestClass("min", array());
$obj6 = new TestClass("fullname");
?>
Another way to overcome PHP's lack of multi constructors support is to check for the constructor's parameters' type and variate the operation accordingly. This works well when the number of parameters stays the same in all your constructors' needs.
Example:
<?php
class Articles {
private $_articles_array;
public function __construct($input) {
if (is_array($input)) {
// Initialize articles array with input.
}
else if ($input instanceof SimpleXMLElement) {
// Initialize articles array by parsing the XML.
}
else {
throw new Exception('Wrong input type');
}
}
}
?>
Please be aware of when using __destruct() in which you are unsetting variables...
Consider the following code:
<?php
class my_class {
public $error_reporting = false;
function __construct($error_reporting = false) {
$this->error_reporting = $error_reporting;
}
function __destruct() {
if($this->error_reporting === true) $this->show_report();
unset($this->error_reporting);
}
?>
The above will result in an error:
Notice: Undefined property: my_class::$error_reporting in my_class.php on line 10
It appears as though the variable will be unset BEFORE it actually can execute the if statement. Removing the unset will fix this. It's not needed anyways as PHP will release everything anyways, but just in case you run across this, you know why ;)
For those who still ***have*** to deal with php4 (like myself, which sucks), contact at tcknetwork dot com noted below that you can have both __constructor and function [classname] in the script, as __constructor has priority.
However, to further improve compatability, do a version check in function [classname], as I do in my classes as follows:
<?php
class Foo{
function __construct($arg){
echo $arg;
}
function Foo($arg){
if(version_compare(PHP_VERSION,"5.0.0","<")){
$this->__construct($arg);
register_shutdown_function(array($this,"__destruct"));
}
}
function __destruct(){
echo "Ack! I'm being destroyed!";
}
}
?>
This makes it much easier to use your script with fewer compatibility issues.
It's usefull to note that parent::__construct() is executed like a standalone function called into the new one, and not like a first part of the code. So "return" or "die()" don't have effect on following code. If you want to avoid the execution of the child constructor you have to pass some condition from the parent. Example:
<?php
class X {
function X(){
if($_POST['break']=='yes'){return;}
}
}
class Y extends X{
function Y(){
parent::__construct();
print("Y_constructor_finished");
}
}
$_POST['break']=='yes';
$new_Y=new Y(); // will print "Y_constructor_finished"
Solution:
class X {
function X(){
if($_POST['break']=='yes'){$this->break='yes';return;}
}
}
class Y extends X{
function Y(){
parent::__construct();
if($this->break=='yes'){return;}
print("Y_constructor_finished");
}
}
$_POST['break']='yes';
$new_Y=new Y(); // will print nothing
?>
I just thought I'd share some valuable information that might help novice programmers/scripters in __constructor() functions.
I've been using PHP for quite a long time, as well as programming and scripting in general. Even with my quite seasoned skillset, I still make obvious, painstaking mistakes. That said, I'd like to point out one major (albeit obvious) common mistake that programmers and scripters (such as myself) make.
When defining the constructor for an object and using it to populate variables for use throughout the lifetime of the application, do note that if you set a static variable AFTER a dynamic one, the static variable's (intended) contents are not readible inside the assigning functions of a dynamic variable. (That sounded rather cryptic, so an example is in order...)
<?php
class ExampleClass
{
// assigned a static value using an expression in the constructor
public $myStaticVar;
// assigned a value through a function of the class
public $myDynamicVar;
public function __construct($myStaticVarValue)
{
// sets to nothing; $this->myStaticVar hasn't been assigned to yet
$this->myDynamicVar = $this->setSomeValue();
// sets to 'Static Variable Value in this example
$this->myStaticVar = $myStaticVarValue;
}
public function setSomeValue()
{
// returns null; $this->myStaticVar hasn't been assigned to, yet
return $this->myStaticVar;
}
}
// automatically sets $object->myStaticVar to 'Static Variable Value'
//(but only after setting $object->myDynamicVar to the unassigned $object->myStaticVar)
$object = new ExampleClass('Static Variable Value');
// outputs nothing
print $object->myDynamicVar;
?>
The above example won't print anything because you're setting the $myDynamicVar to equal the $myStaticVar, but the $myStaticVar hasn't been (technically) assigned to, yet.
So, if you wish to reference that variable inside the assignment function of a dynamic variable, you have to declare it first and foremost. Otherwise, its value will not be accessible yet.
The correct way...
<?php
class ExampleClass
{
// assigned a static value using an expression in the constructor
public $myStaticVar;
// assigned a value through a function of the class
public $myDynamicVar;
public function __construct($myStaticVarValue)
{
// sets to $myStaticVariable (or 'Some Static Variable Value' in this example)
$this->myStaticVar = $myStaticVarValue;
// sets to newly assigned-to $this->myStaticVariable
$this->myDynamicVar = $this->setSomeValue();
}
public function setSomeValue()
{
// returns 'Static Variable Value'
return $this->myStaticVar;
}
}
// sets $object->myStaticVar to 'Static Variable Value' automatically before $object->myDynamicVar
$object = new ExampleClass('Static Variable Value');
// outputs 'Static Variable Value'
print $object->myDynamicVar;
?>
You can also capitalize on singleton for the connection as well.
Just add the following to your class:
<?php
class db {
//...
private static $instance;
//...
public static function singleton() {
if(!isset(self::$instance)) {
$c = __CLASS__;
self::$instance = new $c();
}
return self::$instance;
}
//...
}
?>
It's always the easy things that get you -
Being new to OOP, it took me quite a while to figure out that there are TWO underscores in front of the word __construct.
It is __construct
Not _construct
Extremely obvious once you figure it out, but it can be sooo frustrating until you do.
I spent quite a bit of needless time debugging working code.
I even thought about it a few times, thinking it looked a little long in the examples, but at the time that just seemed silly(always thinking "oh somebody would have made that clear if it weren't just a regular underscore...")
All the manuals I looked at, all the tuturials I read, all the examples I browsed through - not once did anybody mention this!
(please don't tell me it's explained somewhere on this page and I just missed it, you'll only add to my pain.)
I hope this helps somebody else!
Constructor Simplicity
If your class DOES CONTAIN instance members (variables) that need to be set, then your class needs to be initialized... and you should use __construct() to do that.
<?php
class MyClassA {
public $data1, $data2;
public function __construct($mcd1, $mcd2) {
$this->data1 = $mcd1; // INITIALIZE $data1
$this->data2 = $mcd2; // INITIALIZE $data2
}
}
$obj1 = new MyClassA("Hello", "World!"); // INSTANTIATE MyClassA
$d1 = $obj1->data1;
$d2 = $obj1->data2;
?>
If your class DOES NOT CONTAIN instance members or you DO NOT want to instantiate it, then there is no reason to initialize it or use __construct().
<?php
class MyClassB {
const DATA1 = "Hello";
public static $data2 = "World!";
}
$obj1 = new MyClassB(); // INSTANTIATE MyClassB - NO error.
$d1 = $obj1::DATA1; // ERROR
$d2 = $obj1::data2; // ERROR
$d1 = MyClassB::DATA1; // ok
$d2 = MyClassB::$data2; // ok
?>
The fact that $obj1 is useless and cannot be used as a reference, is further evidence that MyClassB objects should not be instantiated. NOTICE that MyClassB does not use private members or functions to make it behave that way. Rather, it is the collective nature of all the class members + what ISN'T there.
USE PARENT::CONSTRUCT() to exploit POLYMORPHISM POWERS
Since we are still in the __construct and __destruct section, alot of emphasis has been on __destruct - which I know nothing about. But I would like to show the power of parent::__construct for use with PHP's OOP polymorphic behavior (you'll see what this is very quickly).
In my example, I have created a fairly robust base class that does everything that all subclasses need to do. Here's the base class def.
<?php
/*
* Animal.php
*
* This class holds all data, and defines all functions that all
* subclass extensions need to use.
*
*/
abstract class Animal
{
public $type;
public $name;
public $sound;
/*
* called by Dog, Cat, Bird, etc.
*/
public function __construct($aType, $aName, $aSound)
{
$this->type = $aType;
$this->name = $aName;
$this->sound = $aSound;
}
/*
* define the sorting rules - we will sort all Animals by name.
*/
public static function compare($a, $b)
{
if($a->name < $b->name) return -1;
else if($a->name == $b->name) return 0;
else return 1;
}
/*
* a String representation for all Animals.
*/
public function __toString()
{
return "$this->name the $this->type goes $this->sound";
}
}
?>
Trying to instantiate an object of type Animal will not work...
$myPet = new Animal("Parrot", "Captain Jack", "Kaaawww!"); // throws Fatal Error: cannot instantiate abstract class Animal.
Declaring Animal as abstract is like killing two birds with one stone. 1. We stop it from being instantiated - which means we do not need a private __construct() or a static getInstance() method, and 2. We can use it for polymorphic behavior. In our case here, that means "__construct", "__toString" and "compare" will be called for all subclasses of Animal that have not defined their own implementations.
The following subclasses use parent::__construct(), which sends all new data to Animal. Our Animal class stores this data and defines functions for polymorphism to work... and the best part is, it keeps our subclass defs super short and even sweeter.
<?php
class Dog extends Animal{
public function __construct($name){
parent::__construct("Dog", $name, "woof!");
}
}
class Cat extends Animal{
public function __construct($name){
parent::__construct("Cat", $name, "meeoow!");
}
}
class Bird extends Animal{
public function __construct($name){
parent::__construct("Bird", $name, "chirp chirp!!");
}
}
# create a PHP Array and initialize it with Animal objects
$animals = array(
new Dog("Fido"),
new Bird("Celeste"),
new Cat("Pussy"),
new Dog("Brad"),
new Bird("Kiki"),
new Cat("Abraham"),
new Dog("Jawbone")
);
# sort $animals with PHP's usort - calls Animal::compare() many many times.
usort($animals, array("Animal", "compare"));
# print out the sorted results - calls Animal->__toString().
foreach($animals as $animal) echo "$animal<br>\n";
?>
The results are "sorted by name" and "printed" by the Animal class:
Abraham the Cat goes meeoow!
Brad the Dog goes woof!
Celeste the Bird goes chirp chirp!!
Fido the Dog goes woof!
Jawbone the Dog goes woof!
Kiki the Bird goes chirp chirp!!
Pussy the Cat goes meeoow!
Using parent::__construct() in a subclass and a super smart base class, gives your child objects a headstart in life, by alleviating them from having to define or handle several error and exception routines that they have no control over.
Notice how subclass definitions are really short - no variables or functions at all, and there is no private __construct() method anywhere? Notice how objects of type Dog, Cat, and Bird are all sorted by our base class Animal? All the class definitions above address several issues (keeping objects from being instantiated) and enforces the desired, consistent, and reliable behavior everytime... with the least amount of code. In addition, new extenstions can easily be created. Each subclass is now super easy to redefine or even extend... now that you can see a way to do it.
Ensuring that instance of some class will be available in destructor of some other class is easy: just keep a reference to that instance in this other class.
Since my last note, I've been instructed to _NEVER_ call "unset($this)", never ever ever. Since my previous testing of behavior did not explicitly show me that it was indeed necessary, I'm inclined to trust those telling me not to do it.
While experimenting with destructs and unsets in relation to memory usage, I found what seems to be one useful way to predict when __destruct() gets called... call it manually yourself.
I had previously assumed that explicitly calling unset($foo) would cause $foo->__destruct() to run implicitly, but that was not the behavior I saw (php 5.2.5). The destructors weren't running until the script ended, even though I was calling unset($foo) in the middle of my script. So, having $foo->__destruct() unset all of $foo's component objects was not helping my memory usage since my explicit unset($foo) was _not_ triggering $foo->__destruct()'s cleanup steps.
Interestingly, what _did_ appear to happen is that calling unset($bar) from inside a destructor like $foo->__destruct() _DID_ cause $bar->__destruct() to be implicitly executed. Perhaps this is because $bar has a "parent" reference of $foo whereas $foo does not, and the object destruction behaves differently... I don't know.
Lastly, even after explicitly calling $foo->__destruct() (even when it had "unset($this);" inside it), the reference to the $foo object remained visible. I had to still explicitly call unset($foo) to get rid of it.
So, my advice based on the behavior I saw in my experiments:
- always unset($bar) your class's component objects from inside that class's __destruct() method; no explicit component destructor calls seem to be required other than those unsets, though.
- always explicitly call $foo->__destruct() in your code that _uses_ your class
- always explicitly follow $foo->__destruct() with unset($foo).
This seems to be the best cleanup approach to take. Just for my own sanity, I also will always keep an unset($this) at the end of each __destruct() method.
Correction to the previous poster about non public constructors. If I wanted to implement Singleton design pattern where I would only want one instance of the class I would want to prevent instantiation of the class from outside of the class by making the constructor private. An example follows:
class Foo {
private static $instance;
private __construct() {
// Do stuff
}
public static getInstance() {
if (!isset(self::$instance)) {
$c = __CLASS__;
$instance = new $c;
}
return self::$instance;
}
public function sayHello() {
echo "Hello World!!";
}
}
$bar = Foo::getInstance();
// Prints 'Hello World' on the screen.
$bar -> sayHello();
When a script is in the process of die()ing, you can't count on the order in which __destruct() will be called.
For a script I have been working on, I wanted to do transparent low-level encryption of any outgoing data. To accomplish this, I used a global singleton class configured like this:
class EncryptedComms
{
private $C;
private $objs = array();
private static $_me;
public static function destroyAfter(&$obj)
{
self::getInstance()->objs[] =& $obj;
/*
Hopefully by forcing a reference to another object to exist
inside this class, the referenced object will need to be destroyed
before garbage collection can occur on this object. This will force
this object's destruct method to be fired AFTER the destructors of
all the objects referenced here.
*/
}
public function __construct($key)
{
$this->C = new SimpleCrypt($key);
ob_start(array($this,'getBuffer'));
}
public static function &getInstance($key=NULL)
{
if(!self::$_me && $key)
self::$_me = new EncryptedComms($key);
else
return self::$_me;
}
public function __destruct()
{
ob_end_flush();
}
public function getBuffer($str)
{
return $this->C->encrypt($str);
}
}
In this example, I tried to register other objects to always be destroyed just before this object. Like this:
class A
{
public function __construct()
{
EncryptedComms::destroyAfter($this);
}
}
One would think that the references to the objects contained in the singleton would be destroyed first, but this is not the case. In fact, this won't work even if you reverse the paradigm and store a reference to EncryptedComms in every object you'd like to be destroyed before it.
In short, when a script die()s, there doesn't seem to be any way to predict the order in which the destructors will fire.
Looking through the notes I noticed a few people expressing concern that PHP5 does not support multiple constructors...
Here is an example of a method that I use which seems to work fine:
class Example
{
function __construct()
{
echo "do some basic stuff here";
}
function Example($arg)
{
echo $arg;
}
}
You then can call with or without arguments without having notices and/or warnings thrown at you... Of course this is limited but if you don't need something complex this can help to get the job done in some situations. I believe you could also add arguments to the __construct() function and as long as it is different than Example() 's args you would be fine. Although I have yet to test this.
i have written a quick example about the order of destructors and shutdown functions in php 5.2.1:
<?php
class destruction {
var $name;
function destruction($name) {
$this->name = $name;
register_shutdown_function(array(&$this, "shutdown"));
}
function shutdown() {
echo 'shutdown: '.$this->name."\n";
}
function __destruct() {
echo 'destruct: '.$this->name."\n";
}
}
$a = new destruction('a: global 1');
function test() {
$b = new destruction('b: func 1');
$c = new destruction('c: func 2');
}
test();
$d = new destruction('d: global 2');
?>
this will output:
shutdown: a: global 1
shutdown: b: func 1
shutdown: c: func 2
shutdown: d: global 2
destruct: b: func 1
destruct: c: func 2
destruct: d: global 2
destruct: a: global 1
conclusions:
destructors are always called on script end.
destructors are called in order of their "context": first functions, then global objects
objects in function context are deleted in order as they are set (older objects first).
objects in global context are deleted in reverse order (older objects last)
shutdown functions are called before the destructors.
shutdown functions are called in there "register" order. ;)
regards, J
The fact that class names are case-insensitive in PHP5 also applies to constructors. Make sure you don't have any functions named like the class *at all*.
This has bitten me a few times.
<?php
class Example extends Base
{
function example()
{
echo "This gets called";
}
}
class Base
{
function __construct()
{
echo "Not this";
}
}
?>
This is just to clarify that the Singleton pattern is a bit more complex than just making the constructor private. It also involves caching an instance of the object and always returning the cached value. So, in the previous example, the getNewInstance() function would undermine the intent of the Singleton pattern. Instead you would just need a getInstance() function, like so.
<?php
class A {
// cached instance
private static oInst = null;
/**
* Prevent an object from being constructed.
*/
private function __construct( ) {}
/**
* Function to return the instance of this class.
*/
public static function getInstance( ) {
if (is_null(self::$oInst)) {
self::$oInst = new A( );
}
return self::$oInst;
}
}
?>
The manual says:
"Like constructors, parent destructors will not be called implicitly by the engine."
This is true ONLY when a __destruct() function has been defined by the child class.
If no __destruct() function exists in the child class, the parent's one will be implicitly executed.
So be carefull if you have some ancestor executing a particular task in its __destruct() function, an you plan its childs to execute it or not, wether you include "parent::__destruct()" or not.
If you want the child not to execute its parent __destruct() function, you must ensure that it has its own __destruct() function, even if empty. Then the parent's one will not be executed.
This can be verified with the following code:
<?php
#
class AncestorClass {
function __destruct() {
echo '<br />AncestorClass: destructing '.get_class($this);
}
}
#
class ParentDestructClass extends AncestorClass {
function __destruct() {
echo 'ParentDestructClass: destructing itself';
parent::__destruct();
}
}
#
class EmptyDestructClass extends AncestorClass {
function __destruct() {
echo 'EmptyDestructClass: destructing itself';
}
}
#
class NoDestructClass extends AncestorClass {
}
#---
echo '<hr>';
$p=new ParentDestructClass();
unset($p);
echo '<hr>';
$e=new EmptyDestructClass();
unset($e);
echo '<hr>';
$n=new NoDestructClass();
unset($n);
echo '<hr>';
?>
which displays:
---
ParentDestructClass: destructing itself
AncestorClass: destructing ParentDestructClass
---
EmptyDestructClass: destructing itself
---
AncestorClass: destructing NoDestructClass
---
Peter has suggested using static methods to compensate for unavailability of multiple constructors in PHP. This works fine for most purposes, but if you have a class hierarchy and want to delegate parts of initialization to the parent class, you can no longer use this scheme. It is because unlike constructors, in a static method you need to do the instantiation yourself. So if you call the parent static method, you will get an object of parent type which you can't continue to initialize with derived class fields.
Imagine you have an Employee class and a derived HourlyEmployee class and you want to be able to construct these objects out of some XML input too.
<?php
class Employee {
public function __construct($inName) {
$this->name = $inName;
}
public static function constructFromDom($inDom)
{
$name = $inDom->name;
return new Employee($name);
}
private $name;
}
class HourlyEmployee extends Employee {
public function __construct($inName, $inHourlyRate) {
parent::__construct($inName);
$this->hourlyRate = $inHourlyRate;
}
public static function constructFromDom($inDom)
{
// can't call parent::constructFromDom($inDom)
// need to do all the work here again
$name = $inDom->name; // increased coupling
$hourlyRate = $inDom->hourlyrate;
return new EmployeeHourly($name, $hourlyRate);
}
private $hourlyRate;
}
?>
The only solution is to merge the two constructors in one by adding an optional $inDom parameter to every constructor.
There were many notes about the inability of defining multiple constructors for the class.
My solution is to define separate static methods for each type of constructor.
<?php
class Vector {
private $x;
private $y;
public function __construct() {
$this->x = 0;
$this->y = 0;
}
public static function createXY($x, $y) {
$v = new Vector();
$v->x = $x;
$v->y = $y;
return $v;
}
}
?>
be careful while trying to access files with __destruct() because the base directory (getcwd()) will be the root of your server and not the path of your script, so add before all your path called in __destruct() :
EITHER dirname($_SERVER["SCRIPT_FILENAME"])."my/path/"
OR dirname(__FILE__)."my/path/"
(be careful with includes, it will give the path of the file processed and not the main file)
It looks like `echo()`ed output from the __destructor() function is displayed onto screen _before_ other output that the class may have have already sent before.
This can be misleading if you have debug info printed in the destructor but not a problem if you know it.
at the end of a script all remaining objects aren't in fact destructed. it is only their __destruct() method, which will be called. the objects still exist after that.
so, if your database connection object has no __destruct() or at least it doesn't disconnects the database, it will still work.
in general, there is no need to disconnect the database (especially for persistent connections).