SELECT (...) FROM User WHERE Username = (...) AND Password = (...);

SELECT COUNT(*) AS `count` FROM `User` WHERE `Username` = ...;

if( $result['count'] < 1 )
{
  // no user
}

// repeat for pass.

SELECT (...) IF(password=?, 1, 0) as success
FROM User
WHERE Username = ?

SELECT password FROM user WHERE username = ?