php sql multi bind_param

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$sql = "SELECT a.COUPONID, a.TRUSTANDUSEID FROM `custom_redemptions` a WHERE a.couponid = 3";

    $types = '';
    $params = array(&$types);

    if ($branchid != null) {
        $sql .= " AND a.branchid = ?";
        $types .= 's';
        $params[] = $branchid;
    }
    if ($chainid != null) {
        $sql .= " AND a.chainid = ?";
        $types .= 's';
        $params[] = $chainid;
    }

    if ($stmt = $this->dbCon->prepare($sql)) {
        call_user_func_array(array($stmt, 'bind_param'), $params);
        $stmt->execute();
        $stmt->bind_result($couponid, $trustanduseid);
        while ($stmt->fetch()) { echo $couponid; }
        $stmt->close();
    }