What is PHPXref?
PHPXref is a developer tool that's designed to ease the process of working on large PHP projects by making it very fast and easy to browse the code documentation along with the code itself.
It works by scanning a project directory and translating the files it finds into readable cross-referenced HTML, simultaneously utilizing comments in the code to produce documentation to accompany it.
The result is a collection of plain HTML files that can be read using any browser, with no supporting software required.
Moodle and PHPXref
For the most up-to-date and detailed description of how the Moodle code works you can browse the code online.
Usually with PHPXref you can search for a class, functions, variables, constants and tables. When searching for a function (eg. get_records) you will reach a page wich lists where it id defined and where it is referenced, giving you a better idea of what it is used for.