If you want to keep doing websites, I'd suggest that you stay with the same technologies, but learn more about them.
You've not really specified what kind of work you've been doing in PHP / HTML / CSS / JavaScript / MySQL, so it's kind of difficult to give pointers.
On the PHP side, there are a number of decent frameworks you could have a look into, such as Zend Framework, or CakePHP.
You might also want to learn about some pre-existing CMS systems, like Joomla or Drupal. There's a lot to go through - the system itself, the add-ons available, and developing your own addons. Unless I'm building something that has no CMS-like features whatsoever (which is rare), I tend to build almost everything on top of Joomla.
There's also improving your PHP skills. It's pretty easy to fall into the trap of writing quick, simple, and unmaintainable spaghetti code with PHP. There are all kinds of useful patterns and design techniques you can pick up (the above-mentioned frameworks or CMSes can provide some hints about how to do it, and how not to do it). You could look into stuff like classes / objects, proper database abstraction layers (PDO, or even ADO), applying design patterns to PHP projects. The standard libraries contain lots of cool functionality you probably haven't used, and the PEAR libraries contain even more.
On the JavaScript side, you might want to look into frameworks like JQuery to start with, and then some of the plugins that have been built on top of it, like JQuery UI. Add some AJAX to your repertoire.
