Rather than try to grab the whole ?xxxx string and process it yourself, just check for the existance of each value and then take action if there IS a value plugged in...
$_GET will work provided your URL is phrased properly and you know what values to expect. No spaces, commas, etc. The line should look like "show.php?id=4212&gameid=1923".
if(isset($_GET['id'])) {
//do something
}
if(isset($_GET['gameid'])) {
//do something
}