$str = "abcdef啊啊吧啊";function my_sub($str, $st ,$len){    $ret = "";    for( $st; $len > 0; $st++){        $cStr = $str[$st];        if( ord( $cStr ) > 127 ){            $cStr = substr($str, $st, 3);            $st += 2;        }        $len -= 1;        $ret .= $cStr;    }    return $ret;} echo my_sub($str, 3, 6);