How can I read others' codes to maximize the gains?

How can I read others' codes to maximize the gains? How can we see that others' codes receive the largest goods? What are the open source code of php's excellent and classic? How can we improve our capabilities?

Reply to discussion (solution)

What are their characteristics?
Many people will look for classes written by someone to study. I feel a headache for this.
I am more interested in others' questions and answers. stackoverflow.com and here is my Resource Library.

What are their characteristics?
Many people will look for classes written by someone to study. I feel a headache for this.
I am more interested in others' questions and answers. stackoverflow.com and here is my Resource Library.
Haha, stackoverflow.com is really good.

In fact, it is quite good to study cms, such as joomla, drupal, and wordpress.

Dedecms has a good idea, especially table structure.