Warning: mysql_query() expects parameter 2 to be resource, object given in

Warning: mysql_query() expects parameter 2 to be resource, object given in /srv/disk2/2584025/www/yogurt.eu5.net/system/login.php on line 15Database error, please try again

não sei onde errei

<?

$rs = mysqli_query("SELECT * FROM users WHERE username='" .$_POST['username'] . "' LIMIT 1", $db)
 or die("Database error, please try again");

	 

	connect.php

<?

$db = mysqli_connect($MYSQL_SERVER,$MYSQL_USERNAME,$MYSQL_PASSWORD);
mysqli_select_db($db, $MYSQL_DB_NAME);

?>