php mysql probem MX: Cannot modify header information

Hi,
suddenly I find myself with the error message
in the "delete" server bahaviour.

it seems to originate from this:
.... snip
$deleteGoTo = "show.php";
if (isset($HTTP_SERVER_VARS['QUERY_STRING'])) {
$deleteGoTo .= (strpos($deleteGoTo, '?')) ? "&" : "?";
$deleteGoTo .= $HTTP_SERVER_VARS['QUERY_STRING'];
}
header(sprintf("Location: %s", $deleteGoTo));
.....snip

I once had found out the problem but for godknowswhynot I cannot figure
out waht it was. BTW I use DW MX (you can see thei from the old php syntax)

Any clue?
thank you