strncmp

(PHP 4, PHP 5, PHP 7)

strncmp — 二进制安全比较字符串开头的若干个字符

说明

strncmp ( string $str1 , string $str2 , int $len ) : int

该函数与 strcmp() 类似，不同之处在于你可以指定两个字符串比较时使用的长度（即最大比较长度）。

注意该比较区分大小写。

参数

str1

第一个字符串。

str2

第二个字符串。

len

最大比较长度。

返回值

如果 str1 小于 str2 返回 < 0； 如果 str1 大于 str2 返回 > 0；如果两者相等，返回 0。

参见

  • strncasecmp() - 二进制安全比较字符串开头的若干个字符（不区分大小写）
  • preg_match() - 执行匹配正则表达式
  • substr_compare() - 二进制安全比较字符串（从偏移位置比较指定长度）
  • strcmp() - 二进制安全字符串比较
  • strstr() - 查找字符串的首次出现
  • substr() - 返回字符串的子串

User Contributed Notes

samy 01-May-2017 01:40
I just want to highlight that (at least on php7), when testing for the existence of a string in the beginning of another string you should consider using substr  or strpos (if performances is an issue).

Here is a small benchmark (for what it's worth):
<?php
$n = 'abcd';
$l = strlen($n);
$haystack0 = base64_encode(random_bytes(128));

//heat
$r = 1;
for ($i = 0; $i < 100000000; $i++)
    $r += $r * $r % 10000;

//tests
$k = 30000000;
$res = array();
foreach (array('found' => $n . $haystack0, 'not-found' => strrev($n) . $haystack0) as $f => $haystack) {
    $m = microtime(true);
    for ($i = 0; $i < $k; $i++)
        !strncmp($haystack, $n, $l) && $r++;
    $res["strncmp-$f"] = -$m + ($m = microtime(true));

    for ($i = 0; $i < $k; $i++)
        (strpos($haystack, $n) === 0) && $r++;
    $res["strpos-$f"] = -$m + ($m = microtime(true));

    for ($i = 0; $i < $k; $i++)
        (substr($haystack, 0, $l) === $n) && $r++;
    $res["substr-$f"] = microtime(true) - $m;
}

//print
asort($res);
print_r($res);
echo "\n$r"; // makes sure no auto-optimization occurs
?>

This outputs:
<?php /*
    [strpos-found]        => 1.3313138484955
    [substr-not-found]    => 1.4832630157471
    [substr-found]        => 1.6976611614227
    [strpos-not-found]    => 2.0043320655823
    [strncmp-not-found]    => 2.0969619750977
    [strncmp-found]        => 2.3616981506348
*/ ?>
salehrezq at gmail dot com 06-Oct-2016 03:42
A note not included in the documentation:

int strcmp ( string $str1 , string $str2 )

Returns < 0 if str1 is less than str2; > 0 if str1 is greater than str2, and 0 if they are equal.

My addendum:
If str1 and str2 are not equal, and str1 is a sub-string of str2 or vise versa. The returned int value will be negative or positive indicating how many characters the difference is between the two strings in absolute terms.

Example:

<?php
$str1 = "phpaaa";
$str2 = "php";

echo strcmp($str1, $str2); // 3
?>

since str2 = "php" is a sub-string of str1 = "phpaaa" and "phpaaa" is greater than "php" the returned value is positive and is 3 indicating how many characters the difference is between the two strings.

If you replace the value of str1 with str2 the result will be -3 (negative) but still indicates the absolute difference which is 3
bobvin at pillars dot net 19-May-2011 07:25
For checking matches at the beginning of a short string, strpos() is about 15% faster than strncmp().

Here's a benchmark program to prove it:

<?php
$haystack = "abcdefghijklmnopqrstuvwxyz";
$needles = array('abc', 'xyz', '123');
foreach ($needles as $needle) {
  $times['strncmp'][$needle] = -microtime(true);
  for ($i = 0; $i < 1000000; $i++) {
    $result = strncmp($haystack, $needle, 3) === 0;
  }
  $times['strncmp'][$needle] += microtime(true);
}
foreach ($needles as $needle) {
  $times['strpos'][$needle] = -microtime(true);
  for ($i = 0; $i < 1000000; $i++) {
    $result = strpos($haystack, $needle) === 0;
  }
  $times['strpos'][$needle] += microtime(true);
}
var_export($times);
?>
elloromtz at gmail dot com 19-Apr-2010 05:25
if length is 0 regardless what the two strings are, it will return 0

<?php
strncmp("xybc","a3234",0); // 0
strncmp("blah123","hohoho", 0); //0
?>
codeguru at crazyprogrammer dot cba dot pl 24-Jan-2008 11:07
I ran the following experiment to compare arrays.

1 st - using (substr($key,0,5 == "HTTP_") & 2 nd - using (!strncmp($key, 'HTTP_', 5))

I wanted to work out the fastest way to get the first few characters from a array

BENCHMARK ITERATION RESULT IS:
if (substr($key,0,5 == "HTTP_").... -   0,000481s
if (!strncmp($key, 'HTTP_', 5)).... -     0,000405s

strncmp() is 20% faster than substr() :D

<?php
// SAMPLE FUNCTION
function strncmp_match($arr)
{
foreach ($arr as $key => $val)
    {
    //if (substr($key,0,5 == "HTTP_")
    if (!strncmp($key, 'HTTP_', 5))   
        {
    $out[$key] = $val;
        }
    }
return $out;
}

// EXAMPLE USE
?><pre><?php
print_r(strncmp_match($_SERVER));
?></pre>

will display code like this:

Array
(
    [HTTP_ACCEPT] => XXX
    [HTTP_ACCEPT_LANGUAGE] => pl
    [HTTP_UA_CPU] => x64
    [HTTP_ACCEPT_ENCODING] => gzip, deflate
    [HTTP_USER_AGENT] => Mozilla/4.0
                                    (compatible; MSIE 7.0;
                                     Windows NT 5.1;
                                    .NET CLR 1.1.4322;
                                    .NET CLR 2.0.50727)
    [HTTP_HOST] => XXX.XXX.XXX.XXX
    [HTTP_CONNECTION] => Keep-Alive
    [HTTP_COOKIE] => __utma=XX;__utmz=XX.utmccn=(direct)|utmcsr=(direct)|utmcmd=(none)
)