I taught myself HTML/CSS, but PHP/MySQL is a whole other ball of wax, especially considering I'd be handling peoples' personal info and possibly CC numbers, etc.
All i want to say is, how you get the knowledge really dose not matter as long as you understand it. And I will also say here are free php lessons http://phpacademy.org/ so if you don't want to pay you could learn there. but in the end you will be the one making the choice and paying the money. So really its up to you alone.