Help for beginner

Hi,
let me start by saying that I am learning so most probably my question will sound quite dump
I am trying to implement into my WordPress page some code from w3school, https://tryphp.w3schools.com/showphp.php?filename=demo_global_post

I have copied the code into PhpCode snippet and result is at https://www.meurelacionamento.tk/loretest/ (my site)

Once I submit it does not appear the name but I am redirected to index.

So 2 questions:
– What I am doing wrong
– Can you point me to a place where I can learn specifically about wordpress and php code

The page I need help with: [log in to see the link]