PHP is relatively easy to learn, even more so if you have experience with basic programming like for and while loops. SQL is the language used to grab things from a database and is super easy to learn, though with PHP you'll be using MySQL, which is very similar.
JavaScript wise, Ender has used jQuery and I'd recommend you do the same. It's a library of JavaScript functions that, once included into a page, makes performing certain tasks like Animations and AJAX calls a LOT easier.
To learn JavaScript, jQuery, and even advance your HTML + CSS skills, I'd recommend Codecademy. It has interactive lessons and is what I used to learn jQuery.
To learn PHP/MySQL I did this course. It's lectures and tasks covered by a Harvard professor. This covers PHP, MySQL, AJAX, JSON.
If you ever get stuck, StackOverflow is a great place to ask any questions you may have, and you'll get many replies very quickly.