Someone

$trivia = simplexml_load_file("trivia.xml");
$vCount = count($trivia);

$vCategory = $_GET['category'];


// Get the topic from the user
if (!ctype_digit($vCategory))
{
	for($i = 0; $i < $vCount; $i++)
	{
		echo("<a href=index.php?category=$i>".$trivia->topic[(integer)$i]->attributes()."</a><br>");
		
	}
}
else
{
   	echo $trivia->topic[(integer)$vCategory]->attributes();
}