What is if isset ($_ post submit )) in php?

I have a little problem with my if[isset[$_POST['submit']]] code. What I want is some echos and a table to not appear when the script is open but I do want it to show when the submit button for the form has been clicked. The problem is though that when I include the if[isset[$_POST['submit']]] function, when I click on the submit button it does not display the echos and the table at all. Why is this and can you help me out with this issue please.

Below is the code:

    



Exam Interface




NOTE: If a search box is left blank, then the form will search for all data under that specific field

        
Session ID: 
      
Module Number: 

  

      
Teacher Username: 
      
Student Username: 
      
Grade: 
      
Order Results By: 
Session ID
Module Number
Teacher Username
Student Username
Grade