Suppose your machine is GMT+1. 

When date('r') returns also GMT+1, strtotime()
shows a bug.

<?
$stamp= "Thu, 20 Dec 2001 09:49:52 +0100";
$stamp= strtotime($stamp);
echo "$stamp = " . date("r", $stamp);                 
//Shows: 1008481792 = Sun, 16 Dec 2001 06:49:52 +0100
?>

To give you an idea of the solution, I use the 
following code to get correct results:

<?
$stamp= "20 Dec 2001 09:49:52 +0100";
// beg patch-line 
if( strpos($stamp, date('O')))$stamp= substr($stamp,0,-6);
// end patch-line 
$stamp= strtotime($stamp);
echo "$stamp = " . date("r", $stamp);                 
//Shows: 1008838192 = Thu, 20 Dec 2001 09:49:52 +0100
?>

There is no problem when GMT date('r') 
is different from GMT as returned by date('O')

Regards, IRMaturana