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

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

第一个字符串。

第二个字符串。

最大比较长度。

返回值

如果$str1小于$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 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:

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):

This outputs:

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

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