連想配列格納　ある記述方法がわからない

前提・実現したいこと

json形式の都道府県のデータをphp連想配列にセットして表示したい。

発生している問題・エラーメッセージ

jsonソースコードから
[
都道府県名 => [札幌市中央区,札幌市北区...]
都道府県名 => [札幌市中央区,札幌市北区...
]
のようにセットして表示する際に
下記のような書き方を調べたら見つけたのですが、

  foreach($jsonDecode[0] as $value){
    foreach($value['city'] as $city){
      $prfAndcity[$value['name']][] = $city['city'];
    }
  }
  var_dump($prfAndcity);

$prfAndcity[$value['name']][] の書き方の意味がわかりません。
空の[ ]は何をあらわしているのでしょうか？
連想配列格納は$prfAndcity[$value['name']で十分だと思うのですが、たしかにこれだとうまくいきませんでした。
どなたかご教示よろしくお願いいたします。

jsonのソースコード

[{
  "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"}
  ]
},
.
.
.
.
"47": {
  "id": "47",
  "name": "沖縄県",
  "short": "沖縄",
  "kana": "オキナワケン",
  "en": "okinawa",
  "city":[
      {"citycode":"0470001","city":"那覇市"},
      {"citycode":"0470002","city":"宜野湾市"},
      {"citycode":"0470003","city":"石垣市"},
      {"citycode":"0470004","city":"浦添市"},
      {"citycode":"0470005","city":"名護市"},
      {"citycode":"0470006","city":"糸満市"},
      {"citycode":"0470007","city":"沖縄市"},
      {"citycode":"0470008","city":"豊見城市"},
      {"citycode":"0470009","city":"うるま市"},
      {"citycode":"0470010","city":"宮古島市"},
      {"citycode":"0470011","city":"南城市"},
      {"citycode":"0470012","city":"国頭郡"},
      {"citycode":"0470013","city":"中頭郡"},
      {"citycode":"0470014","city":"島尻郡"},
      {"citycode":"0470015","city":"宮古郡"},
      {"citycode":"0470016","city":"八重山郡"}
  ]
}
}]

試したこと

ここに問題に対して試したことを記載してください。

補足情報（FW/ツールのバージョンなど）

ここにより詳細な情報を記載してください。

行動規範の内容に同意します

回答1件

ベストアンサー

空の[ ]は何をあらわしているのでしょうか？

配列に新たな要素を追加することを表しています。
マニュアルを読みましょう。
PHP: 配列 - Manual

投稿2021/09/15 15:17

ku__ra__ge

総合スコア4524

coresan

2021/09/16 03:09

うまく伝わっておらず申し訳ありません。 []←が追加の意味は把握しております。自分の考えとしては $prfAndcity[$value['name'] ]= $city['city'] の書き方だけで$value['name']をキーに$city['city']の値を$prfAndcityに格納（これを繰り返し）できると思っています。（実際はできませんでしたが自分の認識はできると思っていた）それを踏まえて $prfAndcity[$value['name']][]= $city['city']は上記とは何が（自分の認識とは）違うのでしょうか。ご教示よろしくお願いします。

ku__ra__ge

2021/09/16 05:12

$x = array('abc','def'); $x = 'ghi'; ……は、$xに'ghi'を代入します。($xにもともと入っていた配列は上書きされて消える) $x = array('abc','def'); $x[] = 'ghi'; ……は、$x配列に'abc'を追加します。($xは['abc','def', 'ghi']になる) それと同じことで、 $prfAndcity[ $value['name'] ] = $city['city']; は、$prfAndcity連想配列の$value['name']キーに、(現在何が入っていようが)$city['city']を上書き代入します。 $prfAndcity[ $value['name'] ][] = $city['city']; は、$prfAndcity連想配列の$value['name']キーに配列が入っていると想定して、その配列に$city['city']の値を追加します。

coresan

2021/09/16 14:23

ku__ra__geさん丁寧な回答ありがとうございました。上記の説明で、全て理解できました。ネットでこの書き方がヒットしなかったので、とてもスッキリしました。どうもありがとうございました。

行動規範の内容に同意します