Something

function rot_x($str,$num=13) {
   $rot  = "";
   for ($i = 0; $i < strlen($str); ++$i) {
     $char = ord($str{$i});
     $cap = $char & 32;
     $char &= ~$cap;
     $char = (($char >= ord('A')) && ($char <= ord('z'))) ? (($char - ord('A') + $num) % 26 + ord('A')) : $char;
     $char |= $cap;
     $rot .= chr($char);
   }
   return $rot;
}