Change it to
PHP Code:
........
require("header.inc")
.......
NB: Changing from include to require will change the type of error generated from a warning (which you may not see) to an actual error message (i.e. the program should crash if it can't include the "header.inc" file)...
If it crashes, it should say why (likely to be can't find the file), you can then correct the path.
If this doesn't help, post some more details + some more code please.