自己写的php中文截取函数mb_strlen和mb_substr

if ( !function_exists('mb_strlen') ) {
 function mb_strlen ($text, $encode) {
  if ($encode=='UTF-8') {
   return preg_match_all('%(?:
       [\x09\x0A\x0D\x20-\x7E]           # ASCII
     | [\xC2-\xDF][\x80-\xBF]            # non-overlong 2-byte
     |  \xE0[\xA0-\xBF][\x80-\xBF]       # excluding overlongs
     | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2} # straight 3-byte
     |  \xED[\x80-\x9F][\x80-\xBF]       # excluding surrogates
     |  \xF0[\x90-\xBF][\x80-\xBF]{2}    # planes 1-3
     | [\xF1-\xF3][\x80-\xBF]{3}         # planes 4-15
     |  \xF4[\x80-\x8F][\x80-\xBF]{2}    # plane 16
     )%xs',$text,$out);
  }else{
   return strlen($text);
  }
 }
}

/* from Internet, author unknown */
if (!function_exists('mb_substr')) {
    function mb_substr($str, $start, $len = '', $encoding="UTF-8"){
        $limit = strlen($str);
 
        for ($s = 0; $start > 0;--$start) {// found the real start
            if ($s >= $limit)
                break;
 
            if ($str[$s] <= "\x7F")
                ++$s;
            else {
                ++$s; // skip length
 
                while ($str[$s] >= "\x80" && $str[$s] <= "\xBF")
                    ++$s;
            }
        }
 
        if ($len == '')
            return substr($str, $s);
        else
            for ($e = $s; $len > 0; --$len) {//found the real end
                if ($e >= $limit)
                    break;
 
                if ($str[$e] <= "\x7F")
                    ++$e;
                else {
                    ++$e;//skip length
 
                    while ($str[$e] >= "\x80" && $str[$e] <= "\xBF" && $e < $limit)
                        ++$e;
                }
            }
 
        return substr($str, $s, $e - $s);
    }
}