これらの数行の何が問題なのかわかりません。
if ( $stmt = $db->prepare('SELECT * FROM users WHERE username = ? LIMIT 1');
$stmt->bind_param('s', $username);
$stmt->execute();
$stmt->store_result();
$stmt->fetch();
$stmt->num_rows == 1;
)
{
$errors[] = "Username is taken.";
}