How does the grammar parser for IntelliJIDEA work?

Question

PHP

I'm trying to write a plugin for the Zephir language .
I am quite familiar with the language, I also roughly figured out the parser.
It is not at all clear where to start, I looked at Dart, erlang, some other not very well-known language, when the parser is added - everything seems to be clear.
In general, the question is basically simple, how to implement the simplest design

namespace foo;
class bar {}

And how to describe lines and comments for ParserDefinition