Access denied for user 'admin'@'localhost' (using password: NO)

First off I connect using this code here.

$mysqli = new mysqli("localhost", $username, $password, $database);

This works fine, I log in under root, (I know I should create a new user for security, but this so far is just a proof of concept) However when I attempt to do a query such as

$sql = "SELECT 
                        user_id,
                        user_name,
                        user_level
                    FROM
                        users
                    WHERE
                        user_name = '" . mysql_real_escape_string($_POST['user_name']) . "'
                    AND
                        user_pass = '" . sha1($_POST['user_pass']) . "'";
                         
            $result = mysql_query($sql);

It gives me the errors message Access denied for user ‘admin’@‘localhost’ (using password: NO). I don’t know why this is happening. Do I need to configure mysql a certain way?