I'm trying to display the username of the person who has submitted the most articles but I don't know how to do it using MySQL & PHP, can someone help me?

Here is the MySQL code.

CREATE TABLE users (
user_id INT UNSIGNED NOT NULL AUTO_INCREMENT,
username VARCHAR(255) DEFAULT NULL,
pass CHAR(40) NOT NULL,
PRIMARY KEY (user_id)
);

CREATE TABLE users_articles (
id INT UNSIGNED NOT NULL AUTO_INCREMENT,
user_id INT UNSIGNED mNOT NULL,
title TEXT NOT NULL,
acontent LONGTEXT NOT NULL,
PRIMARY KEY (id)
);

Here is the code I have so far.

$mysqli = mysqli_connect("localhost", "root", "", "sitename");
$dbc = mysqli_query($mysqli,"SELECT COUNT(*) as coun, user_id 
                             FROM users_articles 
                             GROUP BY user_id 
                             ORDER BY coun DESC
                             LIMIT 1");