URL problem

I've been posting in the install section but I'm not getting the right answers.

The page only refreshes and doesn't do what it's supposed to. It's like it's not even reading the ?add=yes&subtract=no.

I just installed php4.0.3 or some version like that.

Does anybody have any suggestions?

I know the scripts work because I used them on a windows system but I am now on Linux.

here's a quick script that won't work:

<?
if($add == 'yes')
{
print("hi");
}
else
{
print("<a href=\"$PHP_SELF?add=yes\">Add</a>");
}
?>

all it does it refreshes what's currently showing which would be "Add" and never carries out the if($add....

What's going on?

if($_POST['add'] == "yes") {
    echo "hi";
} else {
    echo "<a href=\"$PHP_SELF?add=yes\">Add</a>";
}