Low Variables & PHP Redirect

gistfile1.aw

<?php
     header( "Location: {exp:channel:entries channel='external_link'}{external_link:var}{/exp:channel:entries}" );
     exit;
?>

<!-- This gives me the error: "The URI you submitted has disallowed characters." -->

<!-- And the URL looks like this: http://mysite.com/template_group/%7Blv_twitter%7D -->