primary key = (id + userid) / multiple table joins in PHP Mysql

I am a mediocre programmer, BUT as long as I have a sample to follow (to change the variables) I usually can build websites. I am trying something different, instead of the basic "id" as the primary key, I want the constraint "user-id" added to the primary key: primary key = (id + userid)... (Budget: $10 - $220 USD, Jobs: JavaScript, MySQL, PHP, phpMyAdmin, Software Architecture)