来自POST数据的Webhook URL？

我正在尝试用php编写一个简单的webhook，我想从url中提取一个文件并将其保存在我设置的位置

我得到了一个POST数据示例，它看起来像这样：

{
       "eventType":"OrderGenerated",
       "data":{
          "orderCode":"000000283",
          "orderAmount":7.9,
          "orderDate":"2018-06-08T08:47:30.17",
          "orderDetails":[
             {
                "detailCode":"1912",
                "detailDesignDocId":"xxxxxx-xxxxxx-xxxxx-xxxxx",
                "detailCustomerId":0,
                "detailVisitorId":36542,
                "detailQuantity":1,
                "detailPrice":7.9,
                "detailModelName":"T-Shirt",
                "detailModelCode":"BCTM010",
                "detailFiles":[
                   {
                      "variantName": "Red",
                      "sideName":"Front",
                      "areaID":1,
                      "format":"svg",
                      "fileUrl":"....svg"
                   },
                   ...
                ]
             }
          ]
       }
    }

我想我需要使用“fileUrl”，但我该如何称呼它呢？ 我尝试了$_POST[“fileUrl”]-它似乎不起作用。我该怎么做？谢谢