What does $_GET[‘iphone’] do?

$_GET is for accessing variables in URL parameters, it’s not related to any detection by itself, something must be adding it to URL.

In context of WordPress it might be related to $is_iphone global.

If you look at wp-includes/vars.php there is number of global variables like this, which WP fills up with data via user agent sniffing.

So $is_iphone is set there according to following logic:

if ( $is_safari && stripos($_SERVER['HTTP_USER_AGENT'], 'mobile') !== false )
    $is_iphone = true;