You can lookup built-in functions by searching for e.g. PHPAPI(stream_copy_to_stream) and find the implementation in ext/standard/streamsfuncs.c.
How to do that for a language construct like echo?
I found it is associated with T_ECHO in Zend/zend_language_parser.y but couldn't trace how it works from there.
Where does Zend Engine implement echo functionality?