Step 1) Learn and master HTML (XHTML and HTML5).
Step 2) Learn and master CSS (CSS2 and CSS3).
This step could be optional, but it would really help in the long run.
Step 4) Learn and master PHP (PHP4 (optional), PHP5, and OOP concepts).
Step 5) Learn MySQL (basic PHP mysql() query statements can be enough to get by). Knowledge of of how relationship structure and optimization can (and should) be learned over time.
That’s really about it to begin becoming a web developer expert on LAMP systems. After you have a understanding of how to write your own applications, move into PHP frameworks (like CodeIgniter, CakePHP, or Symfony).