[Adrian Schneider]

PHP Code:

if ($_GET['action'] == 'blah')
{
    echo 'hi';
} 


or

PHP Code:

$vbulletin->input->clean_array_gpc('g', array(
    'do' => TYPE_STR,
    'threadid' => TYPE_INT,
    'action' => TYPE_STR
));

if ($vbulletin->GPC['action'] == 'value')
{
    echo 'eggnog';
} 


If you are going to use the values for anything other than script nav, you should pass it through the gpc functions first to sanitize the data.