Cannot modify header information

["webmaster" <ken.poyser@xxxxxxxxxxx>]

My next problem I am having is 
the  header function....


my command is 

if (!$card_num) {
    header ("Location: get_card.html");
    exit;
  }


and I am get back Cannot modify header information - header is already
by (output stared at ......) in ......  on line ##


I have read something about whitespaces, in the location file,
and it appears that there are no leading whitespaces in the location
file


Any suggestions,
Ken