Here is the code
The code has been obfuscated very slightly, just to provide a bit more fun.
<?php
if (isset($_GET['submit']) && $_GET['submit'] == 'Guess') {
$arrGuess = str_split($_GET['guess']);
$success = false;
if (count($arrGuess) == 16) {
for ($i = 0; $i < 16; $i++) {
$toSolve = ($i < 10) ? $i+48 : $i+55;
$arrSolved[] = ($toSolve == ord($arrGuess[$i])) ? 1 : 0;
if ($arrSolved[$i] == 0) break;
}
if (array_unique($arrSolved) == array(1)) $success = true;
}
echo ($success) ? '<p>Successful guess</p>' : '<p>Unsuccessful guess</p>';
}
?>