Warning when calling mysqli_error() [duplicate]

Answer 1

There are lots of comments on how to fix the mysqli_error line, but there are two problems with the SQL itself:

1. You are joining the users_categories table twice and not joining the users table at all.
2. The second ON is identical to the first, so it references the wrong columns in the wrong tables.

This should work better:

SELECT users.*, categories.*, users_categories.*
FROM users_categories
INNER JOIN users ON users_categories.user_id = users.user_id 
INNER JOIN categories ON users_categories.category_id = categories.id
WHERE users_categories.user_id=3