The easiest (and usual) way is to simply make separate header.php and footer.php files, and access them where you need them. There's no direct support for only loading parts of a file.
Edit (to respond to your comment on the other answer about using separate functions): Let's say your file.php looks like this:
<?php
function header() { ?>
header content goes here
<?php }
function footer() { ?>
footer content goes here
<?php }
?>
Then in the page you're calling it into, you can use <?php header() ?>
and <?php footer() ?>
to produce the content where you want it.
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