Question

I usually do assigning the $_POST superglobal to a variable something like $post to quickly write my code but I started thinking lately whether I am doing is good or not?

Answer 1

I usually write a function for this:

function post($key) {
   return isset($_POST[$key]) ? trim($_POST[$key]) : null;
}

But to answer you question, no there is nothing intrinsically wrong with this. It just has the potential for a lack of clarity and $_POST is in the global scope whereas your assignment is not.