**PHP AJAX** refers to using PHP as the server-side language to handle AJAX requests, while **jQuery AJAX** is a client-side method provided by the jQuery library to simplify making AJAX calls from the browser. In essence, PHP processes the data on the server, and jQuery AJAX is used to send and receive data asynchronously between the client and server.

To integrate AJAX with PHP, first, create an HTML element (like a button) that triggers the AJAX request. Use jQuery or vanilla JavaScript to send an AJAX request to a PHP script, specifying the request type (GET or POST) and any data to send. In the PHP script, process the incoming data, perform any necessary actions (like querying a database), and return a response. Finally, use the AJAX success callback to handle the response and update the webpage dynamically without reloading.

To prevent clients from seeing your PHP/AJAX source code, keep sensitive code on the server side and only expose necessary APIs. Use server-side scripts to handle logic, while AJAX requests retrieve data. Additionally, use minification or obfuscation for JavaScript files, and implement security measures like HTTPS and proper authentication to protect against unauthorized access.

To get data from another URL using PHP, AJAX, and jQuery, use jQuery's `$.ajax()` method to send a request to your PHP script. In the PHP script, use `file_get_contents()` or cURL to fetch data from the external URL. Return the data to the AJAX call, which can then process and display it in the frontend.

To upload files with AJAX using JavaScript and PHP, create a form with an input of type `file`. Use JavaScript to capture the form submission and create a `FormData` object containing the file. Send this data to a PHP script via an AJAX request. The PHP script processes the upload and saves the file to the server.