its not a matter of global variables. gloval variables come into effect when you are working with seperate functions.
But yes you have to use $_GET['variable'] and $_POST['variable']. What i do a lot of times when im programing is right after the <?php set $_GET['variable'] equal to the reular variable so its easier to read.
PHP Code:
<?php
$variable = $_GET['variable'];
//your code
?>