Whatever you choose as editing software, you will actually be able to read all moodle code from a download tarball of Moodle as it is fully open and natively "readable" (not compiled)
I suggest you starting there :
http://docs.moodle.org/dev/Main_Page
As architecture startup knowledge, or you might seek a lot in more than 15 M code lines !
Personally, i tried lot of well-known IDEs, never with so great experience to get rid of my UltraEdit editor. As UltraEdit (ICM) is paid software, an alternative is SUperNotTab, well known by students, that have also powerfull enough code editing oriented tools. What may just lack in those rich pro editors is the code completion, instant help binders, and class navigators, but PHP does not (well, maybe my experience is a bit oldie now....) allow often real "code navigation" experience suvh in Java because of the dynamic source linking by include/require references.... (script tehnique related issue)