Rot13

function RotThirteen($input)
{
    $allowed_chars = array_merge(range('a','z'), range('A','Z'));
    array_push($allowed_chars, " ");

    $output = "";
    for($i = 0; $i < strlen($input); $i++) {
        $current_character = $input[$i];

        if ( !in_array($current_character, $allowed_chars) ) {
            continue;
        }

        $ascii_code = ord($current_character);
        $rot13_code = $ascii_code + 13;

        $substitute_character = "";
        if ( $current_character == " " ) {
            $substitute_character = $current_character;
        } else {
            if ( $rot13_code > 122 ) {
                $rot13_code = 97 + ($rot13_code - 123);
            } else if ( $rot13_code > 90 && $rot13_code < 104) {
                $rot13_code = 65 + ($rot13_code - 91);
            }

            $substitute_character = chr($rot13_code);
        }

        $output .= $substitute_character;
    }

    return $output;
}