выключить Register_Globals.

<?php

// URL: http://example.com/test.php?my_var=1
// register_globals = On

// define "my_var" variable here to prevent the attacks
$my_var = "";

echo ($my_var != "") ? "my_var has a value: ".$my_var : "my_var doesn't have any value";
//returns "my_var doesn't have any value" in any case (On/Off)!

// let's check the superglobal $_GET["my_var"] variable
$my_var2 = $_GET["my_var"];
echo '<br />$_GET["my_var"] has a value: '.$my_var2;

// Note: it's better to use the new "inner" variable like $my_var2 in your scripts
// try to avoid to use the superglobal variables and to assign their's values to the new ones

?>

<?php

// URL: http://example.com/test.php?my_var=1
// register_globals = On

// my_var doesn't have the predefined value

echo ($my_var != "") ? "my_var has a value: ".$my_var : "my_var doesn't have any value";
// returns "my_var has value: 1"

// let's check the superglobal $_GET["my_var"] variable
$my_var2 = $_GET["my_var"];
echo '<br />$_GET["my_var"] has a value: '.$my_var2;

// Note: it's better to use the new "inner" variable like $my_var2 in your scripts
// try to avoid to use the superglobal variables and to assign their's values to the new ones

?>