Warning: mysqli_connect(): (HY000/1045): Access denied for user 'username'@'localhost' (using password: YES)

That combination of username, host, and password is not allowed to connect to the server. Verify the permission tables (reloading grants if required) on the server and that you're connecting to the correct server.

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

mysqli_connect(DB_SERVER,DB_USERNAME,DB_PASSWORD,DB_DATABASE);

use DB_HOST instead of DB_SERVER