I would say it depends on your experience with Web application languages. First, I would get better by learning the HTTP protocol and everything related (client side VS server side, how does the HTTP protocol works, cookies, etc.)
Now, for back-end development, you could learn a bit more about Web services like REST with PHP.
You could also learn a couple of lightweight frameworks like Laravel and Codeigniter. I don't include Drupal or WP in the "framework" group, those are CMS platforms IMO.
Also, you could read this Website:
http://www.phptherightway.com/
It's not complete but there are a lot of subjects that you could deepen by yourself (security, OO programming, etc.).
After you're more comfortable with those advanced subject, it will be time to delve into topics like performance testing, scalability and architecture.
Good luck! Don't hesitate if you have any other questions