EDIT

<?php

$logoutURL = array(

  'http://your.host.org/drupal5/?q=logout',

  'http://your.host.org/drupal6/?q=logout'

);

$ch = curl_init();

curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);

curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);

curl_setopt($ch, CURLOPT_HEADER, 1);

$cookies='';

foreach ($_COOKIE as $key => $value) {

  $cookies= $cookies . $key . '=' . $value.'; ';

}

curl_setopt($ch, CURLOPT_COOKIE, $cookies);

foreach ( $logoutURL as $key => $url ) {

	curl_setopt($ch, CURLOPT_URL, $url);

	$doc=curl_exec ($ch);

	$info = curl_getinfo( $ch);

}

curl_close ($ch);

header('Location: ' . $_REQUEST['return'], TRUE, 302);

exit();

?>