PHP JSONの多次元配列から値を連結して取得したい

Question

閲覧ありがとうございます。

JSONファイルを読み取り、多次元配列から
47都道府県名に対する市をすべて表示させたいのですが
エラー内容「Notice: Undefined index: city in /work/index.php on line 37」と表示されてしまします。

以下の型式で表示するにあたりコードへのアドバイスを頂けませんでしょうか。

$cityArrayと$count
$cityArray .=",". $val["city"]["city"];
等の記述は配列の結合について以下のページに記載があったので参考にしています。

https://alaki.co.jp/blog/?p=1776

表示させたい型式
～～～～～～～～～～～～～～～～～
[
"都道府県名A" => [市町村名1, 市町村名2...],
"都道府県名B" => [市町村名3, 市町村名4...],
...
]
～～～～～～～～～～～～～～～～～

$file = "./sample.json"; //jsonファイルへのパス
$json = file_get_contents($file); //ile_get_contents関数を用いて、$fileの内容を文字列として読み込む
$json = mb_convert_encoding($json, 'UTF8', 'ASCII,JIS,UTF-8,EUC-JP,SJIS-WIN');
$arr = json_decode($json,true);//$jsonの内容を連想配列とし$arrに格納する
$count = 0;
$cityArray = "";
 
foreach($arr[0] as $key => $val){
  if($count == 0){
    $cityArray .= $val["city"]["city"];
  }else{
    $cityArray .=",". $val["city"]["city"];
  }
  $count++;
};

var_dump($cityArray);

[{
  "01": {
  "id": "01",
  "name": "北海道",
  "short": "北海道",
  "kana": "ホッカイドウ",
  "en": "hokkaido",
  "city":[
      {"citycode":"0010001","city":"札幌市中央区"},
      {"citycode":"0010002","city":"札幌市北区"},
      {"citycode":"0010003","city":"札幌市東区"},
      {"citycode":"0010004","city":"札幌市白石区"},
      {"citycode":"0010005","city":"札幌市豊平区"},
      {"citycode":"0010006","city":"札幌市南区"},
      {"citycode":"0010007","city":"札幌市西区"},
      {"citycode":"0010008","city":"札幌市厚別区"},
      {"citycode":"0010009","city":"札幌市手稲区"},
      {"citycode":"0010010","city":"札幌市清田区"},
      {"citycode":"0010011","city":"函館市"},
      {"citycode":"0010012","city":"小樽市"},
      {"citycode":"0010013","city":"旭川市"},
      {"citycode":"0010014","city":"室蘭市"},
      {"citycode":"0010015","city":"釧路市"},
      {"citycode":"0010016","city":"帯広市"},
      {"citycode":"0010017","city":"北見市"},
      {"citycode":"0010018","city":"夕張市"},
      {"citycode":"0010019","city":"岩見沢市"},
      {"citycode":"0010020","city":"網走市"},
      {"citycode":"0010021","city":"留萌市"},
      {"citycode":"0010022","city":"苫小牧市"},
      {"citycode":"0010023","city":"稚内市"},
      {"citycode":"0010024","city":"美唄市"},
      {"citycode":"0010025","city":"芦別市"},
      {"citycode":"0010026","city":"江別市"},
      {"citycode":"0010027","city":"赤平市"},
      {"citycode":"0010028","city":"紋別市"},
      {"citycode":"0010029","city":"士別市"},
      {"citycode":"0010030","city":"名寄市"},
      {"citycode":"0010031","city":"三笠市"},
      {"citycode":"0010032","city":"根室市"},
      {"citycode":"0010033","city":"千歳市"},
      {"citycode":"0010034","city":"滝川市"},
      {"citycode":"0010035","city":"砂川市"},
      {"citycode":"0010036","city":"歌志内市"},
      {"citycode":"0010037","city":"深川市"},
      {"citycode":"0010038","city":"富良野市"},
      {"citycode":"0010039","city":"登別市"},
      {"citycode":"0010040","city":"恵庭市"},
      {"citycode":"0010041","city":"伊達市"},
      {"citycode":"0010042","city":"北広島市"},
      {"citycode":"0010043","city":"石狩市"},
      {"citycode":"0010044","city":"北斗市"},
      {"citycode":"0010045","city":"石狩郡"},
      {"citycode":"0010046","city":"松前郡"},
      {"citycode":"0010047","city":"上磯郡"},
      {"citycode":"0010048","city":"亀田郡"},
      {"citycode":"0010049","city":"茅部郡"},
      {"citycode":"0010050","city":"二海郡"},
      {"citycode":"0010051","city":"山越郡"},
      {"citycode":"0010052","city":"檜山郡"},
      {"citycode":"0010053","city":"爾志郡"},
      {"citycode":"0010054","city":"奥尻郡"},
      {"citycode":"0010055","city":"瀬棚郡"},
      {"citycode":"0010056","city":"久遠郡"},
      {"citycode":"0010057","city":"島牧郡"},
      {"citycode":"0010058","city":"寿都郡"},
      {"citycode":"0010059","city":"磯谷郡"},
      {"citycode":"0010060","city":"虻田郡"},
      {"citycode":"0010061","city":"岩内郡"},
      {"citycode":"0010062","city":"古宇郡"},
      {"citycode":"0010063","city":"積丹郡"},
      {"citycode":"0010064","city":"古平郡"},
      {"citycode":"0010065","city":"余市郡"},
      {"citycode":"0010066","city":"空知郡"},
      {"citycode":"0010067","city":"夕張郡"},
      {"citycode":"0010068","city":"樺戸郡"},
      {"citycode":"0010069","city":"雨竜郡"},
      {"citycode":"0010070","city":"上川郡"},
      {"citycode":"0010071","city":"勇払郡"},
      {"citycode":"0010072","city":"中川郡"},
      {"citycode":"0010073","city":"増毛郡"},
      {"citycode":"0010074","city":"留萌郡"},
      {"citycode":"0010075","city":"苫前郡"},
      {"citycode":"0010076","city":"天塩郡"},
      {"citycode":"0010077","city":"宗谷郡"},
      {"citycode":"0010078","city":"枝幸郡"},
      {"citycode":"0010079","city":"礼文郡"},
      {"citycode":"0010080","city":"利尻郡"},
      {"citycode":"0010081","city":"網走郡"},
      {"citycode":"0010082","city":"斜里郡"},
      {"citycode":"0010083","city":"常呂郡"},
      {"citycode":"0010084","city":"紋別郡"},
      {"citycode":"0010085","city":"虻田郡"},
      {"citycode":"0010086","city":"有珠郡"},
      {"citycode":"0010087","city":"白老郡"},
      {"citycode":"0010088","city":"沙流郡"},
      {"citycode":"0010089","city":"新冠郡"},
      {"citycode":"0010090","city":"浦河郡"},
      {"citycode":"0010091","city":"様似郡"},
      {"citycode":"0010092","city":"幌泉郡"},
      {"citycode":"0010093","city":"日高郡"},
      {"citycode":"0010094","city":"河東郡"},
      {"citycode":"0010095","city":"河西郡"},
      {"citycode":"0010096","city":"広尾郡"},
      {"citycode":"0010097","city":"足寄郡"},
      {"citycode":"0010098","city":"十勝郡"},
      {"citycode":"0010099","city":"釧路郡"},
      {"citycode":"0010100","city":"厚岸郡"},
      {"citycode":"0010101","city":"川上郡"},
      {"citycode":"0010102","city":"阿寒郡"},
      {"citycode":"0010103","city":"白糠郡"},
      {"citycode":"0010104","city":"野付郡"},
      {"citycode":"0010105","city":"標津郡"},
      {"citycode":"0010106","city":"目梨郡"}
  ]
},
"02": {
  "id": "02",
  "name": "青森県",
  "short": "青森",
  "kana": "アオモリケン",
  "en": "aomori",
  "city":[
      {"citycode":"0020001","city":"青森市"},
      {"citycode":"0020002","city":"弘前市"},
      {"citycode":"0020003","city":"八戸市"},
      {"citycode":"0020004","city":"黒石市"},
      {"citycode":"0020005","city":"五所川原市"},
      {"citycode":"0020006","city":"十和田市"},
      {"citycode":"0020007","city":"三沢市"},
      {"citycode":"0020008","city":"むつ市"},
      {"citycode":"0020009","city":"つがる市"},
      {"citycode":"0020010","city":"平川市"},
      {"citycode":"0020011","city":"東津軽郡"},
      {"citycode":"0020012","city":"西津軽郡"},
      {"citycode":"0020013","city":"中津軽郡"},
      {"citycode":"0020014","city":"南津軽郡"},
      {"citycode":"0020015","city":"北津軽郡"},
      {"citycode":"0020016","city":"上北郡"},
      {"citycode":"0020017","city":"下北郡"},
      {"citycode":"0020018","city":"三戸郡"}
  ]
},
"03": {
  "id": "03",
  "name": "岩手県",
  "short": "岩手",
  "kana": "イワテケン",
  "en": "iwate",
  "city":[
      {"citycode":"0030001","city":"盛岡市"},
      {"citycode":"0030002","city":"宮古市"},
      {"citycode":"0030003","city":"大船渡市"},
      {"citycode":"0030004","city":"花巻市"},
      {"citycode":"0030005","city":"北上市"},
      {"citycode":"0030006","city":"久慈市"},
      {"citycode":"0030007","city":"遠野市"},
      {"citycode":"0030008","city":"一関市"},
      {"citycode":"0030009","city":"陸前高田市"},
      {"citycode":"0030010","city":"釜石市"},
      {"citycode":"0030011","city":"二戸市"},

　以下略

Comments

[m.ts10806]

１つ前の質問の回答が理解できていれば、解決可能ではないでしょうか。

Answer 1

var_dump関数を使って、PHPにデコードしたjsonがどのように収まっているか確認してください。

それから文字の連結にはimplodeを使いましょう。その方法はトラブル発生の元です。