I'm new-ish to php so I don't really know what I'm doing

I'm using moodle for part of a site I'm making
I like how the breadcrumbs work and want to use that same code to cary over to the other part of the site that is not moodle

I've found the variable that defines the breadcrumbs
but I can't seem to find out where the file that it's refrencing to know how the variable work is.
I've looked through the page and don't see anything that directs me to a file like that. How do I go about finding it?


Also the other section of the site that I would be using the breadcumbs on is currently all .html files. So to use php I imagine i'd need to change them to .php. Or is that not nessessary?
What steps are nessessary to change the file extension name?
Do I just change the extension or are there header things I need similar to the < html> and < /html> tags?