As a web developer it is important to have client and server side skills. But there are few web developers which are excellent at both... But knowing the basics and where to look for info is definitely a good starting point.
I would recommend to work through the following list(s) in the order presented. If you feel comfortable with a topic, move one. Even though that in reality you will often have to go back and forth during the learning process...
Client Side
- HTML / XHTML
- CSS
- a JavaScript Library (e.g. jQuery, prototype/scriptaculos, etc.)
Server Side - Infrastructure
- familiarize yourself with shell scripting if working on Linux (e.g. bash, etc.)
- Webserver (e.g. Apache)
- PHP Server
- Databases (e.g. MySQL, PosgreSQL, etc.)
Server Side - Programming
Advanced Topics
After getting familiar with the basics...
- AJAX (this is placed here because it is a client/server topic, not because it is advanced...)
- Object Oriented Programming (OOP)
- Security issues
- Source Control Management (SCM) -> (e.g. git, etc.)
- Model View Controller (MVC) software architecture
- PHP Frameworks (Zend Framework, Symphony, CakePHP, Agavi)
- Unit Testing (e.g. PHPUnit)
- Object Relational Mapper (ORM) for PHP (doctrine, Propel)
Usually this keeps you busy for a while. At least it kept and still keeps me busy... ;)
