php 如何解析json數組

數據庫中的字段裏面放的是json數據格式，
例如：$str=[{"attachment":"images\/5\/2019\/08\/ol777omFo7B2b7N977Bs9m6d6VL5bl.jpg","title":"14.jpg","cover":"0"},{"attachment":"images\/5\/2019\/08\/wzkI8P581TsPMRaM11zm5KAgaH8taL.jpg","title":"12.jpg","cover":"0"},{"attachment":"images\/5\/2019\/08\/R1r5o5Ww17VrLr2lAaR73yLyOl5im7.jpg","title":"13.jpg","cover":"0"}]
現在想將裏面對應的鍵值取出，方法：

      $img= json_decode($str);
       foreach ($img as $value) {
           $imgarr=$value->attachment;    //取出對應的值
           $arr[]=$imgarr;      //將取出的值放入一個數組
       }