/* Initialize an array for filtered data. */
$clean = array();
/* Define a salt. */
define('SALT', 'flyingturtle');
/* Encrypt the password. */
$encrypted_password = md5(SALT . $_POST['password']);
/* Allow alphanumeric usernames. */
if (ctype_alnum($_POST['username'])) {
$clean['username'] = $_POST['username'];
} else {
/* Error */
}
/* Store user in the database. */
$st = $db->prepare('INSERT
INTO
users (username, password)
VALUES (?, ?)');
$st->execute(array($clean['username'], $encrypted_password));