Warning: mysqli_connect(): (HY000/1045): Access denied for user \'username\'@\'localhost\' (using password: YES) in C:\\Users\\xampp\\htdocs\\PHP_Login_Script
Try the above answers 90% of the time this will be you issue. For me my problem was that my database password had a $ in it.
$servername = '127.0.0.1';
$username = 'user_bob';
$password = 'sdfasdf$B';
$conn = mysqli_connect($servername, $username, $password, 'any_database');
//DO NOT USE DOUBLE QUOTES;
//$password = "sdfasdf$B";