Process only sections of a file? (newbie)

Gary <[email protected]>
Newsgroups gmane.comp.gnu.m4.general
Message-ID <[email protected]> 
Sorry if this is obviously documented somewhere, but I couldn't find it
(or perhaps found it and didn't understand it...).

What I would like to do is only have m4 process certain sections of a
file delimited by some given sequence (in my case outside <?php ... ?>
tags). Is that possible?
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