variable dissappearing after SESSION_REGISTER()

I have made a validation script to allow access to my site from a mail link.

The code the user recives is:
...verifima.php?webourl=info.php&uid=password&email=some@body.ct

(but sometimes it will header to an article using $id and $op)

where: $uid have the password,
       $email user email,
       $webourl the url where the script headers.

the script is this:
-------------verifima.php--------------------------
<?  
$conexio = mysql_connect("localhost","","");
    mysql_select_db ("database", $conexio) OR die ("No es pot conectar");
   
    $sql="SELECT email, pwd, id FROM usuarios WHERE email LIKE '".$email."' AND pwd LIKE '".$uid."'";
    $result=mysql_query($sql);

    IF ($row= mysql_fetch_array($result))
    {
      $validat = 'si';
      session_register('validat');
      $origen="mailing";
      session_register('origen');
      $usuari=$row[id];
      session_register('usuari');
      $sesio=SID;
      $op=str_replace(" ","+",$op);
       if ($webourl)
       {
         header ("Location: ".urldecode($webourl)."");
       }
       echo $webourl;
      header ("Location: article.php?op=$op&check[]=$id");
   }
   ELSE
   {
    header ("Location: index.htm");
   }
?>
--------------------EndOfFile------------------

the variable $webourl exists and you can 'echo' it, but after SESSION_REGISTER() it dissappears. And goes to 'article.php'; but if you refresh the page (in the same browser window) you go to 'info.php' (I have tried with IE,Comunicator and Mozilla).
any hint of how to fix it? $op don't dissapear when it is passed.Start Free Trial