I mean:
General forum will show header.php
xxx forum will show header_xxx.php
But forum xxx will not show header.php's content ,
If I want the global header (header.php) also shown in header_xxx.php , I'll have to duplicate header.php's content to header_xxx.php

If I modify header.php , I'll have to modify header_xxx.php , it will increase the complexity of future maintenance.

I want header.php truely be a global header , also shown in the xxx forum

English is not my native language. I try my best to express my thought precisely. I hope you understand what I mean. If any misunderstanding results from culture gaps, I apologize first.