Method $_GET[] PHP

Question

I am using the $_GET[] method in php and it is returning the following error: "Do not access the direct Superglobal Array $_GET directly."

follows the code:

<?php 
  function COMP($Comp, $Lar, $Alt){
    echo $Vol =$Comp*$Lar*$Alt;                            
}
  $Comp = isset($_GET['COMP'])?$_GET['COMP']:0;
  $Lar = isset($_GET['LAR'])?$_GET['LAR']:0;
  $Alt = isset($_GET['ALT'])?$_GET['ALT']:0;

  COMP($Comp, $Lar, $Alt);                        
?>

Thanks for the help.