Why do I need the isset() function in php?

Question

I am trying to understand the difference between this:

if (isset($_POST[\'Submit\'])) { 
  //do something
}

and

if ($_POST[         


        

Answer 1

if user do not enter a value so $_post[] return NULL that we say in the description of isset:"

isset will return TRUE if it exists and is not NULL otherwise it is FALSE.,but in here isset return the true "