How do I find out which user entered the most articles and then count how many articles that user entered using PHP & MySQL.

Here is my MySQL tables.

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

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)
);