Warning: mysqli_connect(): (HY000/1045): Access denied for user \'username\'@\'localhost\' (using password: YES) in C:\\Users\\xampp\\htdocs\\PHP_Login_Script
If youre running wamp update
define("DB_HOST", "localhost");
To your machines ip address (mine is 192.168.0.25);
define("DB_HOST", "192.168.0.25");
You can find it on window by typing ipconfig in your console or ifconfig on mac/linux