How to make a code generator?

Question

PHP

Good afternoon. How do code generators work? Let's say I want to create an index.php with I store templates somewhere `classTemplate.php` open a file with `file_get_contents` and insert variables `className` and so on? Or just doing somethingclass Name extends Event

ClassBuilder->setClassName('Name')->setExtends('Event')

? And how do I paste the formatted code into the file I want to create?
This is a general question, I just don't understand how it works.