<?php
//assuming your file is named index.php
  //index.php without user_criteria shows site 2
  ///index.php?user_criteria=tdorg shows site 1
$serverURL = (isset($_GET['user_criteria']) && $_GET['user_criteria'] == 'tdorg')?"http://trumpdonald.org/":"https://giphy.com/gifs/z48aJruaX0Jsk/embed";
//if the user criteria is set in the URL, use that.  Otherwise, default to url 1
?>
<iframe src="<?php echo $serverURL; ?>" width="100%" height="100%" />