How to Monitor PHP processes?

First I apologize if there is something in this forum about this already, but I couldn’t find it. I’m a graphic designer by training and self taught in coding, so this might be a basic thing. IDK. I couldnt find an answer on Google either.

The long story
I’m trying to use FPDF with a plugin to create PDFs with information generated by the plugin. I’ve rendered a PDF using a page template, so I know WP is capable of doing this and it’s actually a pretty easy thing to do. When I try to render a PDF using the plugin, I am getting an output error from FPDF. Obviously some process is happening preventing FPDF from making an output. I need a way to monitor what is going on behind the scenes, so I know where to stop the process to allow FPDF to render a PDF.

The short story
I need a way to monitor what is going on behind the scenes in the PHP, so I know where to stop a process.

thanks,
-eric