strncmp

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

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 arrayBENCHMARK ITERATION RESULT IS:if (substr($key,0,5 == "HTTP_").... -   0,000481sif (!strncmp($key, 'HTTP_', 5)).... -     0,000405sstrncmp() is 20% faster than substr() :D<?php// SAMPLE FUNCTIONfunction strncmp_match($arr){foreach ($arr as $key => $val)    {    //if (substr($key,0,5 == "HTTP_")    if (!strncmp($key, 'HTTP_', 5))            {    $out[$key] = $val;        }    }return $out;}// EXAMPLE USE?><pre><?phpprint_r(strncmp_match($_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))

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);
?>

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) - $m;}//printasort($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*/ ?>

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
?>