The approach to look at at other programmers code is definitely a good way to learn "good" or "bad" practices. After reading other code you develop a "feeling" for good code.
I definitely recommend Frameworks like Zend Framework, symfony, CakePHP or Agavi because a lot of contributors look at that code and develop them constantly towards the "ideal" solution (which of course will never be reached...) ;) You still can make the decision developing your own classes instead the ones the framework offers...
With such a long PHP background you could consider contributing to an open source framework. Discussions with other developers bring you forward in a faster way.
Showing your code to others and ask for constructive feedback has the same effect. You never know how good you are until you benchmark your skills with others. Always in a constructive manner. This would be a natural process when contributing to an open source framework. ;)
