Add a loading gif while php runs server side

I would like to request a walk through of how to add a loading gif while a php script is running, called from a POST event.
I have found various ways and suggestions in my searches, but (as usual with the web) the solutions I could understand did not work and I don't monkey with code I can't follow!

Like so many things in the IT world, I expected there to be a concise answer. What I found is 100 answers, most of them questionable proposed by a gaggle of "experts" who can't agree on the color of the sky, let alone the answer to the question... (and of course the ubiquitous, "why would you want to do that?")

If one of you could kindly lend your expertise, I would dearly love to learn how to accomplish this. Not just because it is a task I set for myself, but also because I want to learn (correctly!). Thank you!