Gustek

Then why You have in Your code before each query to categories table $mysqli->select_db("categories"); and articles $mysqli->select_db("articles"); ?

What I can see in Your code is that You have each table in separate database? You should have one database with many tables so You don't have to use select_db before each query.

Use $mysqli->prepare(...) or die($mysqli->error); Why do You have separate databases for articles and categories?