Warning: mysqli_connect(): (HY000/1045): Access denied for user \'username\'@\'localhost\' (using password: YES) in C:\\Users\\xampp\\htdocs\\PHP_Login_Script
Look at your code You defined DB_HOST and querying DB_SERVER. DB_USER and DB_USERNAME Use this code
define("DB_SERVER", "localhost");
define("DB_USER", "root");
define("DB_PASSWORD", "");
define("DB_DATABASE", "databasename");
$connect = mysqli_connect(DB_SERVER , DB_USER, DB_PASSWORD, DB_DATABASE);