php字符串截取（兼容中英文混截）

//兼容中英文混排的字符串截取
function common_substr($sourcestr, $cutlength) {
   $returnstr = '';
   $i = 0;
   $n = 0;
   $str_length = strlen ( $sourcestr ); //字符串的字节数 
   while ( ($n < $cutlength) and ($i <= $str_length) ) {
    $temp_str = substr ( $sourcestr, $i, 1 );
    $ascnum = ord ( $temp_str ); //得到字符串中第$i位字符的ascii码 
    if ($ascnum >= 224) {//如果ascii位高与224，
        if($n+3>$cutlength){
            return $returnstr;
        }
        $returnstr = $returnstr . substr ( $sourcestr, $i, 3 ); //根据utf-8编码规范，将3个连续的字符计为单个字符  
        $i = $i + 3; //实际byte计为3
        $n +=3;
        //echo $n."\n";
    } elseif ($ascnum >= 192){ //如果ascii位高与192，
        if( $n+2>$cutlength){
            return $returnstr;
        }
        $returnstr = $returnstr . substr ( $sourcestr, $i, 2 ); //根据utf-8编码规范，将2个连续的字符计为单个字符 
        $i = $i + 2; //实际byte计为2
        $n +=2;
    } elseif ($ascnum >= 65 && $ascnum <= 90) {//如果是大写字母，
        $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
        $i = $i + 1; //实际的byte数仍计1个
        $n ++; //但考虑整体美观，大写字母计成一个高位字符
    }elseif ($ascnum >= 97 && $ascnum <= 122) {
        $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
        $i = $i + 1; //实际的byte数仍计1个
        $n ++; //但考虑整体美观，大写字母计成一个高位字符
    } else {//其他情况下，半角标点符号，
        $returnstr = $returnstr . substr ( $sourcestr, $i, 1 );
        $i = $i + 1; 
        $n = $n + 1;
    }
   }
   return $returnstr;
}