How does one go about debugging PHP?

How does one go about debugging PHP?
I have an open source CMS php web site. The template / theme that I have has some issues. I have tried looking online for a solution along the expected and typical paths but they have reached a dead end.
So I have decided to do it the hard way and actually get into the code and find the error
Any suggestions on how I should begin?

See original post by xarzu