I really am unsure what you all (Daniel/KCK) mean but you can include anything, anyway basically like this :
example #1 :
<? include 'somefile.inc'; ?>
<b>hello, i am html</b>
<p>I am html</p>
<br>
<p><? include 'hi.inc'; ?></p>
example #2 :
<head><? include 'head.inc'; ?></head>
Now, the include files can include anything yes. If you want the include files to parse as PHP you'll want to put <? and ?> within them. So :
example of hi.inc (with php):
<?
echo "<p>hello there <b>$name</b></p>";
?>
example of hi.inc (without php):
<p>hello there <b>silly</b>.</p>
Perhaps that is what you mean? Give us some examples of code and ask "can i do this?" so we can help.