It seems that mod_headers directives Header and RequestHeade have the same functionality. It also seems that the only diference is that Header can read header information sent by PHP, while RequestHeader can not. is that right?
The documentation for Header says:
This directive can replace, merge or remove HTTP response headers. The header is modified just after the content handler and output filters are run, allowing outgoing headers to be modified.
The documentarion for RequestHeader says:
This directive can replace, merge, change or remove HTTP request headers. The header is modified just before the content handler is run, allowing incoming headers to be modified.
It's not right. The difference is request headers vs. response headers.