ajaxjavascriptPHP
I have been into web development with both client amd server side code, and have been using JavaScript Ajaxto receive and manipulate for data along with PHP scripts.
However, I noticed that websites like Google and Facebook use PHP forms instead of sending ajax requeststo PHP scripts.
What I wonder is that what is the advantage of direct PHP form submit over sending form data from a static HTML page to a PHP Script via JavaScript Ajax, especially from the UX and security point of view?
At least your part about the programming language isn't very correct. While what you describe is one (of many) options, describing a web programming language as something "that mimics a traditional desktop application" is leaving out far too much (and the frontend part that makes the web page itself run is mostly done with JavaScript).
Most programming languages used for the Web where initially not even developed for this purpose (PHP maybe the only exception). Ajax isn't a programming language at all, Perl, Ruby and Python are scripting languages and used for many purposes, including traditional desktop applications.
Their purpose if used as programming languages for web app backends is to receive requests from web sites, process the parameters that came with that request and render a response or send files. Often as a web page with css, html, javascript, but also by providing data in form of other formats like json or XML
So things are quite a bit more complex here.
It's not so much "fake" performance as real responsiveness. There are a number of reasons why it's popular:
Best Answer
What are PHP forms? Do you mean HTTP POST requests (Wikipedia, MDN)?
Ajax will usually POST as well. The only difference is that Ajax runs in the background, and does not navigate to a new URL. That also answers the UX question: if the user intention is not to leave that page, a background request seems more sensible.
There are no special security considerations for either case. PHP can be used both securely and insecurely, but this does not depend on the type of request.