in any form that might call your function page include a hidden field. for the sake of the argument call it "what" (or add it as a parameter in your call). on your function page have a switch that uses $_GET['what'] for an argument.
so as part of a form:
<input type="hidden" name="what" value="function1" />
or as part of your call:
and as for getpage.php:
$what = $_GET['what'];
do something else;
all that said, i don't know how jquery fires that off... it probably won't like you using a get string in a post query... so you'll have to sort that detail. but the theory is there.