$.get() is the way to go, indeed.

First of all, you will need a page / url that outputs the result of the function you want to use (for example, *www.yoursite.com/test_output.php* ). You should create that page and call the function you want to use there.
Also keep in mind that I said output, not return, because .get will fetch the output of the http response, not the returned value of the php function.

So if you have the following function defined in your site (you can also define it in test_output.php, of course):

<?php 

function say_hello() {

 return 'hello world';

}

?>

In test_output.php, you will need something like this:

<?php

echo say_hello();

?>

Then on the client side, you need some JavaScript / jQuery:

var data_from_ajax;



$.get('http://www.yoursite.com/test_output.php', function(data) {

  data_from_ajax = data;

});

Now you have the output of the ajax .get() function stored in data_from_ajax and you can use it as you please.