I still ask why? It is obsolete, you are wasting your time learning things from the past which will no longer be used (by anyone who knows what they are doing) in the present or the future.
The project you have set yourself is very ambitious for a beginner, you have a huge amount to learn. I don’t see the point in wasting your time learning obsolete functions and out-dated security methods that you should never be using.
Your time would be much better spent learning current best practice, things that you will actually use in a project.
In all honesty, I think your first step should be to learn HTML and become competent in that before even considering learning the more advanced topic of PHP.
Take it one small step at a time and avoid legacy code at all cost.