Variable question...

Can anyone please tell me why this does not work:

$result = pg_exec($c1,"SELECT o_plz FROM ort_tabelle WHERE o_plz =
$HTTP_SESSION_VARS['user_tabelleXu_o_plz']");

And I have to do the following instead?

$plz = $HTTP_SESSION_VARS['user_tabelleXu_o_plz'];
$result = query_trans($c1, "SELECT o_plz FROM ort_tabelle WHERE o_plz =
$plz");

Tia again Chris