Object di dalam object

$data_adult;
            if(isset($adult_name)){
                if(count($adult_name) > 0){
                    foreach($adult_name as $k => $a){
                        $data_adult['adult'][$k]['name']      = $adult_name[$k];
                        $data_adult['adult'][$k]['phone']     = $phone[$k];
                        $data_adult['adult'][$k]['idNumber']  = $adult_id_number[$k];
                        $data_adult['adult'][$k]['birthdate'] = $adult_birthdate[$k];
                    }
                    $data['passengers'] .= json_encode($data_adult);
                }
            }
            $data_child;
            if(isset($child_name)){
                if(count($child_name) > 0){
                    for($k = 0; $k<count($child_name); $k++) {
                        $data_child['children'][$k]['name']      = $child_name[$k];
                        $data_child['children'][$k]['birthdate'] = $child_birthdate[$k];
                    }
                    $data['passengers'] .= json_encode($data_child);
                }
            }
            $data_infrant = [];
            if(isset($infrant_name)){
                if(count($infrant_name) > 0){
                    foreach($infrant_name as $k => $a){
                        $data_infrant["infants"][$k]['name']      = $infrant_name[$k];
                        $data_infrant['infants'][$k]['birthdate'] = $infrant_birthdate[$k];
                    }
                    $data['passengers'] .= json_encode($data_infrant);
                }
            }