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 $string1, string $string2, int $length): 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

string1: 第一个字符串。
string2: 第二个字符串。
length: 最大比较长度。

返回值

如果 string1 小于 string2，则返回小于 0 的值；如果 string1 大于 string2，则返回大于 0 的值；如果它们相等，则返回 0。除了它的符号外，不能从返回值中可靠推断出任何特定的含义。

更新日志

版本	说明
8.2.0	当字符串长度不相等时，此函数不再保证返回 `strlen($string1) - strlen($string2)`，而可能返回 `-1` 或 `1`。

示例

示例 #1 strncmp() 示例

<?php

$var1 = 'Hello John';
$var2 = 'Hello Doe';
if (strncmp($var1, $var2, 5) === 0) {
    echo 'First 5 characters of $var1 and $var2 are equal in a case-sensitive string comparison';
}
?>

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

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＋添加备注

用户贡献的备注 5 notes

down

salehrezq at gmail dot com ¶

8 years ago

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

down

codeguru at crazyprogrammer dot cba dot pl ¶

16 years ago

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)
)

down

bobvin at pillars dot net ¶

13 years ago

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);

?>

down

samy ¶

7 years ago

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
*/ ?>

down

elloromtz at gmail dot com ¶

14 years ago

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

?>