custom impact mysql error

customimpactsnippet.php

    public function getTotalDonationImpactOfUserIds($user_ids_array){

        $queryGZ = "SELECT SUM(amount) FROM gz_donations where ";
        $queryMGF = "SELECT SUM(amount) FROM mgf_donations where ";

                for ($i=0; $i<count($user_ids_array); $i++){
                        $id = $this->db->escape($user_ids_array[$i]);

                        if ($i == count($user_ids_array) - 1) {
                                $queryGZ = $queryGZ . "referrer_user_id ='$id' OR donated_by_user_id ='$id'";
                                $queryMGF = $queryMGF . "referrer_user_id ='$id' OR donated_by_user_id ='$id'";
                        } else {
                                $queryGZ = $queryGZ . "referrer_user_id ='$id' OR donated_by_user_id ='$id' OR ";
                                $queryMGF = $queryMGF . "referrer_user_id ='$id' OR donated_by_user_id ='$id' OR ";
                        }
                }

                $resultsGZ = $this->db->query($queryGZ);
                $rowGZ = mysql_fetch_assoc($resultsGZ);
                $sumGZ = $rowGZ['SUM(amount)'];

                //CPBLog::debug("GZ sum: " . $sumGZ);

                $resultsMGF = $this->db->query($queryMGF);
                $rowMGF = mysql_fetch_assoc($resultsMGF);
                $sumMGF = $rowMGF['SUM(amount)'];

                //CPBLog::debug("MGF sum: " . $sumMGF);

                return $sumMGF + $sumGZ;
    }