While handling of global variables had the focus on to be easy in PHP 3 and early versions of PHP 4, the focus has changed to be more secure. While in PHP 3 the following example worked fine, in PHP 4 it has to be unset(unset($GLOBALS["id"]));. This is only one issue of global variable handling. You should always have used $GLOBALS, with newer versions of PHP 4 you are forced to do so in most cases. Read more on this subject in the global references section.
Example#1 Migration of global variables
$id = 1;
echo($id); // This will print out 1 in PHP 4