/**
* ROT13 is a simple function which rotates the
* characters of a string 13 places forward.
*
* @param     string    $string         Input string
* @return    string     Encrypted string
*/
function Rot13($string){

        for($i=0; $i < strlen($string); $i++) {

                $c = ord($string[$i]);

                if ($c >= ord('n') & $c <= ord('z') | $c >= ord('N') & $c <= ord('Z'))
                $c -= 13;
                elseif ($c >= ord('a') & $c <= ord('m') | $c >= ord('A') & $c <= ord('M'))
                $c += 13;

                $string[$i] = chr($c);

        }

        return $string;
}