strncmp

(PHP 4, PHP 5, PHP 7, PHP 8)

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)

)