AJAX stands for Asynchronous JavaScript and XML, and it is a technique for creating fast and dynamic web pages. It allows web pages to be updated asynchronously by exchanging small amounts of data with the server behind the scenes, which means that it is possible to update parts of a web page without reloading the whole page. AJAX is based on internet standards and uses a combination of technologies such as the XMLHttpRequest object, JavaScript/DOM, CSS, and XML.

PHP is a server-side scripting language used for generating dynamic page content. AJAX is often used with PHP to create more interactive applications. When a user interacts with a web page, JavaScript makes a request to the server, which is interpreted by PHP, and the server sends back a response that is displayed on the web page.

In summary, AJAX is a technique for creating fast and dynamic web pages, while PHP is a server-side scripting language used for generating dynamic page content. AJAX and PHP are often used together to create more interactive web applications.