strncmp()
(PHP 4, PHP 5, PHP 7)
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
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 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 */ ?>
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) )
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); ?>
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 ?>