PHP-O-Lait, in a few lines of code (three if you include the 'require_once') provides a transparent bridge between server-side PHP methods and client-side JavaScript code.
PHP-O-Lait uses Ajax to send the request and receive the response, and JSON for the data encoding / decoding.
You don't need to know what either of these are to use PHP-O-Lait, but Ajax is a bag-of-tricks using client-side JavaScript to communicate with a server without loading a new page in the browser, and JSON is a light-weight data transfer protocol that is much more bandwidth-friendly than XML, better suited to loosely-typed languages such as PHP and JavaScript.