Introduction to the PHP source code

Hi everyone! I’d like to get familiar with the PHP C code base so that I can contribute back to the community and maybe even implement a small RFC at some point when I feel comfortable enough. Looking at the giant repository I feel a little overwhelmed. Is there a place where I can get started and learn more about the architecture of the interpreter before diving into the source code itself? I’m sorry if this question has been asked before. Any tips would be greatly appreciated. Thank you! Regards, Ilija