Forced Code/Header Insertion/Injection

Hi there,

I'm not really sure how to phrase my question, but i'll try, here goes...

I'm looking for a way to force apache to load content in a header fashion
before serving files on a system.

That is, let's imagine a typical web request scenario.
User goes to www.example-fake-domain.com
Index.php is the default document in this users home folder.
Index.php is served normally.

What I need is something as follows:
User goes to www.example-fake-domain.com
Apache includes a banner at the top of the page
Index.php is the default document in this users home folder.
Index.php is server below the automatically inserted banner.

I *know* this is possible, as I've investigated compromised machines that
somehow injected javascript code into all served pages. I just need to know
if this is possible from an advertisement point of view, similar to what
they do at Geocities etc.

I need this ideally to be handled from Apache, because that way users will
not be able to circumvent it easily or if at all.

Thanks in advance,
Rich.