Call PHP Function using jQuery AJAX

I've read all the topics about my question but cannot solve my problem. I want to get php function result using jQuery AJAX.

function.php

function generateCode() {
//code here
}

function generateResult() {
//code here
}

How can I catch the function result using jQuery AJAX? I don't want to change function.php structure because it related to another pages.

Refer to using jquery $.ajax to call a PHP function, no answer can solve my problem. Thank You

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To use PHP classes and functions nearly directly, you can use xajax.