Concurrent AJAX call

I am attempting to organize my code in the following manner:

    if (is_ajax()) {
        if (isset($_POST["action"]) && !empty($_POST["action"])) { //Checks if action value exists
            $action = $_POST["action"];
            switch($action) { //Switch case for value of action
               case "initMsg": initMsg_function(); break;
               case "initCombo": initCombo_function(); break;
            }
        }
    }

    function is_ajax() {
        return isset($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest';
    }

Whereby every ajax call invokes a method (business logic) associated to the call.

By any chance will my code cause any concurrency issue?

Your help is kindly appreciated.

Thank You.