字符串函数
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strncmp

(PHP 4, PHP 5)

strncmp — 二进制安全比较字符串开头的若干个字符

说明

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

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，返回负数；如果 str1 大于 str2，返回正数；二者相等则返回 0。

参见

字符串函数
在线手册：中文 英文
PHP手册
PHP手册 - N: 二进制安全比较字符串开头的若干个字符

用户评论:

bobvin at pillars dot net (19-May-2011 03:25)

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 01:25)

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 07:07)

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)
)

Anonymous (17-Apr-2002 12:46)

strncmp("sample","sam",4) returns 1 because the final requirement is if one string terminates before len, then the other must also terminate at that position. 

You can imagine that all your strings have one more final, invisible "termination" character.  If that termination character happens to be within in len, then it must match, too.

For instance, write that termination character with, say, the sequence "\0". Then you can equivalently consider that function call as strncmp("sample\0","sam\0",4).

So, the "p" in "sample" does not match the termination character in "sam".