I think this:
$updateSQL = sprintf("UPDATE members SET currentcount = currentcount + '1' WHERE mID=%s",
GetSQLValueString($_POST['nothing'], "int"),
GetSQLValueString($_POST['mID'], "int"));
should be:
$updateSQL = sprintf("UPDATE members SET currentcount = currentcount + '1' WHERE mID=%s",
GetSQLValueString($_POST['mID'], "int"));
Is the mID value set correctly in a hidden field? Why not use the Session directly? Also is the form method POST?