$sth = $pdo->prepare("INSERT INTO `users` SET `username` = :username, `password` = :password");
$sth->execute(array('username' => $username, 'password' => $password);
$sth->execute(array('username' => $username, 'password' => $password);
$sth = $pdo->prepare("INSERT INTO `users` SET `username` = :username, `password` = :password");
$sth->execute(array(':username' => $username, ':password' => $password);