Using an if statement to prove a condition

I'm using an if statement to prove that a condition exists in the database. I want the code to run if the database contains the same username and prompt the user to input another one. This is my if statement. The code below the if statement works, so there's no need to place it here. Am I approaching this correctly?

example:

  if($_POST['username'] == $_POST['username'] )
  {
  My code execution code here. 
  .
  .
  .
  }