PHP实现mb_substr函数

 1 <?php
 2     function myMbSubstr($str, $start, $length, $charset){
 3         $charsets["utf-8"] = $charsets["utf8"] = "/[\x01-\x7f]|[\xc2-\xdf][\x80-\xbf]|[\xe0-\xef][\x80-\xbf]{2}|[\xf0-\xff][\x80-\xbf]{3}/";
 4         $charsets["gb2312"] = "/[\x01-\x7f]|[\xb0-\xf7][\xa0-\xfe]/";
 5         $charsets["gbk"] = "/[\x01-\x7f]|[\x81-\xfe][\x40-\xfe]/";
 6         $charsets["big5"] = "/[\x01-\x7f]|[\x81-\xfe]([\x40-\x7e]|\xa1-\xfe])/";
 7         
 8         preg_match_all($charsets[$charset], $str, $matches);    #按照指定编码将字符串切分成一个数组
 9         
10         #var_dump($matches);
11         
12         $substr = implode("", array_slice($matches[0], $start, $length));    #获取数组的子数组,并将子数组的元素连接起来
13         
14         return $substr;
15     }
16     
17     $str = "I'm a 码农";
18     
19     echo "<br>" . myMbSubstr($str, 0, 7, "utf8") . "<br>";
20     echo mb_substr($str, 0, 7, "utf8");
21 ?>

输出

array(1) { [0]=> array(8) { [0]=> string(1) "I" [1]=> string(1) "'" [2]=> string(1) "m" [3]=> string(1) " " [4]=> string(1) "a" [5]=> string(1) " " [6]=> string(3) "码" [7]=> string(3) "农" } }
I'm a 码
I'm a 码