Using JavaScript/jQuery with Php?

You wouldn't access your PHP variable and functions in your client-side JavaScript. The codes are running on two different machines. After your JavaScript validates the form on the browser, the AJAX call sends the form to the server. So, instead of calling the code directly, you're sending a message to the server that you have a request that needs to be processed and stored. After the PHP takes care of that request, it sends a message back in response.

This is my code so far:

checkForm - is function in jq to check validation of form

        $("#add_task").submit(function(evt) {
            evt.preventDefault();

            if (checkForm) {
                var url = $(this).attr("action");
                $.ajax(url, {
                    method : "POST",
                    data : $("#add_task").serialize(),
                    success : function(response){
                        // NEED TO RUN FUNCTION IN PHP - HOW
                    }
                });
            }
        });

So for each call of ajax I'll need a seperate file of php code to handle it? There is no other way to manage all ajax call in one file??? how?

The PHP code runs on the server and outputs an pure HTML page (or other data formats, but lets stick to HTML). Once that page is fully output and sent to the browser, the PHP code is done. The only way to execute more is to make a new request to the server.