What is the usage of include,require, require_once,header?

Question

I just saw usage of include and require. But don't see header and require_once.

Answer 1

header has a special work, it's not like include, require and require_once
it add response headers and send to client(be sure use it before sending the response, it's against the http rules to send headers before response body)
require_once is like require but if file was alreadg required, the require once has no effect