private function clean_string($string, $strip_tags = true,$empty_null = true) {
    if (!strlen($string) and $empty_null) return NULL;
    if($strip_tags)
      $string = strip_tags($string);
  
    return get_magic_quotes_gpc() ? mysql_escape_string(stripslashes($string)) : mysql_escape_string($string);
  }

This is most likely faster because you're not changing the memory so frequently. In this is_numeric isn't needed because it'd just slow things down. It's really good practice to use mysql_real_escape_string, but I respect this way for simplicity's sake.