(PHP 4, PHP 5, PHP 7)
define — 定义一个常量
成功时返回 TRUE， 或者在失败时返回 FALSE。
Example #1 定义常量
<?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // 输出 "Hello world."
echo Constant; // 输出 "Constant" 并导致 Notice
define("GREETING", "Hello you.", true);
echo GREETING; // 输出 "Hello you."
echo Greeting; // 输出 "Hello you."
// PHP 7 起就可以运行了
define('ANIMALS', array(
'dog',
'cat',
'bird'
));
echo ANIMALS[1]; // 输出 "cat"
?>
It may be worth stating that a define function must be executed before its global constant is referenced.
Example:
Abc();
define("TEST", 23);
function Abc()
{
echo TEST;
} // Abc
This code fails with a Notice-level message. TEST is treated here as being the string "TEST".
You can define constants with variable names (works also with constant values or variables or array values or class properties and so on - as long it's a valid constant name).
<?php
# Define a constant and set a valid constant name as string value
define("SOME_CONSTANT", "NEW_CONSTANT");
# Define a second constant with dynamic name (the value from SOME_CONSTANT)
define(SOME_CONSTANT, "Some value");
# Output
echo SOME_CONSTANT; // prints "NEW_CONSTANT"
echo "<br>";
echo NEW_CONSTANT; // prints "Some value"
?>
Needless to say that you'll lose your IDE support for refactoring and highlighting completely for such cases.
No clue why someone would / could actually use this but i thought it's worth mentioning.
In PHP 5 it seems it was possible to redefine predefined constants under certain circumstances. The following code will output 42 in PHP 5:
<?php
namespace NS;
define('TRUE', 42, false);
echo TRUE;
?>
In PHP 7 the output is different (1).
Not sure why the docs omit this, but when attempting to define() a constant that has already been defined, it will fail, trigger an E_NOTICE and the constant's value will remain as it was originally defined (with the new value ignored).
(Guess that's why they're called "constants".)
With php 7 you can now define arrays.
consider the following code:
<?php
define( "EXPLENATIVES", [1 => "Foo Bar", 2 => "Fehw Bahr", 3 => "Foo Bahr", 4 => "Fooh Bar", 5 => "Fooh Bhar", 6 => "Foo Barr", 7 => "Foogh Bar", 8 => "Fehw Barr", 9 => "Fu bar", 10 => "Foo Bahr", 11 => "Phoo Bar", 12 => "Foo Bawr", 13 => "Phooh Baughr", 14 => "Foogan Bargan", 15 => "Foo Bahre", 16 => "Fu Bahar", 17 => "Fugh Bar", 18 => "Phou Baughr"]);
//set up define methods using mixed values; both array and non-array values
define("NAVBTNS", [EXPLENATIVES, "Nouns", "Verbs", "Adjectives"]);
//function to create a dropdown menu using the EXPLENATIVES array $btn=EXPLENATIVES=assoc_array
function construct_navbar_buttons(){
$btns = '<ul class="nav navbar-nav">';
foreach(NAVBTNS as $button => $btn){
if(is_array($btn)){
$btns .= '<li class="dropdown">
<a class="dropdown-toggle" data-toggle="dropdown" href="?id='.$btn.'">
<i class="glyphicon glyphicon-user"></i> You Dare Say? <i class="glyphicon glyphicon-collapse-down"></i>
</a>
<ul class="dropdown-menu dropdown-user">';
foreach(EXPLENATIVES as $key => $button){
$btns .= '<li><a href="#">'.$button.'</a></li>';
}
$btns .= '</ul>';
}else{
$btns .= '<li><a href="#">'.$btn.'</a></li>';
}
}
$btns .= '</ul>';
return $btns;
}
Love this new implementation!
?>
There's an undocumented side-effect of setting the third parameter to true (case-insensitive constants): these constants can actually be "redefined" as case-sensitive, unless it's all lowercase (which you shouldn't define anyway).
The fact is that case-sensitive constants are stored as is, while case-insensitive constants are stored in lowercase, internally. You're still allowed to define other constants with the same name but capitalized differently (except for all lowercase).
<?php
// "echo CONST" prints 1, same as "echo const", "echo CoNst", etc.
define('CONST', 1, true);
echo CONST; // Prints 1
define('CONST', 2);
echo CONST; // Prints 2
echo CoNsT; // Prints 1
echo const; // Prints 1
// ** PHP NOTICE: Constant const already defined **
define('const', 3);
echo const; // Prints 1
echo CONST; // Prints 2
?>
Why would you use this?
A third party plugin might attempt to define a constant for which you already set a value. If it's fine for them to set the new value, assuming you cannot edit the plugin, you could define your constant case-insensitive. You can still access the original value, if needed, by using any capitalization other than the one the plugin uses. As a matter of fact, I can't think of another case where you would want a case-insensitive constant...
You can also define associative array as constant:
<?php
define("DB", [
"DB_HOST_NAME" => "localhost",
"DB_USER_NAME" => "root",
"DB_PASSWORD" => "password",
"DB_DATABASE" => "db_example"
]);
prin_r(DB);
Array ( [DB_HOST_NAME] => localhost [DB_USER_NAME] => root [DB_PASSWORD] => password [DB_DATABASE] => db_example )
Php 7 - Define: "Defines a named constant at runtime. In PHP 7, array values are also accepted."
But prior PHP 7, you can maybe do this, to pass an array elsewhere using define:
$to_define_array = serialize($array);
define( "DEFINEANARRAY", $to_define_array );
... and so ...
$serialized = DEFINEANARRAY; // passing directly the defined will not work
$our_array = unserialize($serialized);
print_r($our_array);
I think worth mentioning is that define() appears to ignore invalid constant names.
One immediate implication of this seem to be that if you use an invalid constant name you have to use constant() to access it and obviously that you can't use the return value from define() to tell you whether the constant name used is invalid or not.
For example:
$name = '7(/!§%';
var_dump(define($name, "hello")); // outputs bool(true)
var_dump(constant($name)); // outputs string(5) "hello"
Be aware that if "Notice"-level error reporting is turned off, then trying to use a constant as a variable will result in it being interpreted as a string, if it has not been defined.
I was working on a program which included a config file which contained:
<?php
define('ENABLE_UPLOADS', true);
?>
Since I wanted to remove the ability for uploads, I changed the file to read:
<?php
//define('ENABLE_UPLOADS', true);
?>
However, to my surprise, the program was still allowing uploads. Digging deeper into the code, I discovered this:
<?php
if ( ENABLE_UPLOADS ):
?>
Since 'ENABLE_UPLOADS' was not defined as a constant, PHP was interpreting its use as a string constant, which of course evaluates as True.
Some people like to have an ambiguous, engine-agnostic database include specified by assigning a single config variable to one of a series of constants. Unfortunately, this can easily become needlessly clunky if these defines are handled in an included config.php file, since more than one hook will throw an ugly "already defined" error.
Here's a simple way to accomplish this architectural model without having to create a bunch of clumsy sanity checks that compromise scalability:
<?php
/* Available databases. --Kris */
$dbengines = array();
$dbengines[] = "mysql";
$dbengines[] = "mysqli";
$dbengines[] = "pgsql";
foreach ( $dbengines as $engine )
{
if ( !defined( "_" . strtoupper( $engine ) . "_" ) )
{
define( "_" . strtoupper( $engine ) . "_", $engine );
}
}
/* Which database engine shall we use? --Kris */
$sql_type = _MYSQLI_;
?>
Then, somewhere later in the stack....
<?php
/* Include the file with the desired sql class. --Kris */
require_once( $sql_type . ".php" );
/* Now you can use the same code for any supported db engine. --Kris */
$sql = new sql();
$res = $sql->query( .... );
To clear up a few thing:
Integers with 0 in front work. But since PHP (and many other languages) handle them as octal values, they're only allowed a range of 0-7:
<?php
define('GOOD_OCTAL', 0700);
define('BAD_OCTAL', 0800);
print GOOD_OCTAL;
print '<br>';
print BAD_OCTAL;
?>
Result:
448
0
---------------------------------------
Furthermore,
writing the constant name without the quotation-marks (as mentioned in the notes) throws an E_NOTICE and should be avoided!
<?php
define(TEST, 'Throws an E_NOTICE');
?>
Result:
Notice: Use of undefined constant TEST - assumed 'TEST'
This is obvious, but easy to forget: if you include a file, the include file can only make use of constants already defined. For example:
<?php
define("VEG","cabbage");
require("another file");
define("FRUIT","apple");
// "another file":
echo VEG; // cabbage
echo FRUIT; // FRUIT
?>
The value of a constant can be the value of another constant.
<?php
define("NEW_GOOD_NAME_CONSTANT", "I have a value");
define("OLD_BAD_NAME_CONSTANT", NEW_GOOD_NAME_CONSTANT);
echo NEW_GOOD_NAME_CONSTANT; // current
echo OLD_BAD_NAME_CONSTANT; // legacy
?>
define() will define constants exactly as specified. So, if you want to define a constant in a namespace, you will need to specify the namespace in your call to define(), even if you're calling define() from within a namespace. The following examples will make it clear.
The following code will define the constant "MESSAGE" in the global namespace (i.e. "\MESSAGE").
<?php
namespace test;
define('MESSAGE', 'Hello world!');
?>
The following code will define two constants in the "test" namespace.
<?php
namespace test;
define('test\HELLO', 'Hello world!');
define(__NAMESPACE__ . '\GOODBYE', 'Goodbye cruel world!');
?>
Found something interesting. The following define:
<?php
define("THIS-IS-A-TEST","This is a test");
echo THIS-IS-A-TEST;
?>
Will return a '0'.
Whereas this:
<?php
define("THIS_IS_A_TEST","This is a test");
echo THIS_IS_A_TEST;
?>
Will return 'This is a test'.
This may be common knowledge but I only found out a few minutes ago.
[EDIT BY danbrown AT php DOT net: The original poster is referring to the hyphens versus underscores. Hyphens do not work in defines or variables, which is expected behavior.]
For translating with variables and define, take also a look on the constant() function.
Example :
<?php
define('PAYMENT_IDEAL',"iDEAL Payment ( NL only )");
define('PAYMENT_MASTERCARD',"Mastercard Payment ( international )");
$payparam='MASTERCARD';
echo constant("PAYMENT_$payparam");
// output :
// Mastercard Payment ( international )
?>