it all depends on excellence of the programmer. Merely knowing php does not make anyone earn. Its always a skill to know about how, when and where to apply php. Domain knowledge is also important. Yes, php developer has growth and can earn very well..
You need to have experience in the right areas. If you are looking for decent pay and work, I would learn the Zend framework really well, and have a really good understanding of OOP, and MVC style programming.
If you can prove that you have successfully handled complicated projects, you will be paid more. Just taking tests or reading books isn't usually that interesting to prospective employers (unless they themselves have no knowledge of programming).