Riddle: Bit Play #1

<?php
/**
 * This is a PHP riddle, the object of the game is to guess the correct answer
 * based on the code provided, WITHOUT looping through all the possible answers.
 * The code can be tricky to read, however is completely valid.
 *
 * As you decipher the code below, there will be various hints to help you
 * determine the answer. Example: the code may require an answer with a specific
 * number of caracters.
 *
 *
 *
 * RULES:
 *  - You may run this through the PHP compiler up to 3 times before you are
 *    "wrong"
 *  - You may use an online luhn calculator, and php.net; but no other sites
 *  - You may not add any valid code to this file (comments and whitespace
 *    characters are welcome)
 *  - HARDCORE MODE: Both the internet and localhost are off limits for anything
 *    but confirming your answer
 *  - HARDCORE MODE: You only get 2 tries to get it "right"
 */


/**
 * Riddle: Bit Play #1
 * -------------------
 * This riddle may move, add, remove, compair, and evaluate bit values. It is
 * suggested that you brush up on your binary and bitwise operators before
 * starting.
 */
$answer = 0; // put your answer here and run to check if you are right


// The Setup
function luhn($number) {
    settype($number, 'string');
    $sumTable = array(
        array(0,1,2,3,4,5,6,7,8,9),
        array(0,2,4,6,8,1,3,5,7,9));
    $sum = 0;
    $flip = 0;
    for ($i = strlen($number) - 1; $i >= 0; $i--) {
        $sum += $sumTable[$flip++ & 0x1][$number[$i]];
    }
    return $sum % 10 === 0;
}
settype($answer, 'integer');
$s = (string) $a = &$answer;
$ns = (string) $n = (isset($s[1])) ? ($a >> 1)+$s[1] : ($a >> 1)+$s[0];
$f = ((int) $ns[0] ^ (int) $s[1]);

// The Riddle
function riddle($a, $n, $s, $f, $ns) {
    if (
        ($a != 0 && luhn($a)) &&
        (!$f) &&
        (($n | $a) == 63) &&
        (!isset($s[2]))
    ) {
        return TRUE;
    }
    return FALSE;
}

// The Result
if (riddle($a, $n, $s, $f, $ns)) {
    echo "You got the correct answer! ($a)";
} else {
    echo "Sorry, wrong answer. ($a)";
}