如何获取多维json obj的属性

Question

我正致力于使用jira仪表板中的数据创建刻录图表。在php中使用curl，我得到了一个json对象（如下所示），我存储为$ res。这是多维的，并且作为JSON的新手，我努力将诸如changes，statisticField，startTime，endTime等属性提取到我的php代码中，即使在引用许多答案之后，我发现这太嵌套了。我可以知道如何将这些提取到php变量中吗？

我的json obj是：

    {
    changes: {
    1435802352000: [
    {
    key: "PDM-6680",
    column: {
    notDone: true,
    newStatus: "1"
    }
    }
    ],

    1465224095000: [
    {
    key: "PDM-7104",
    added: true
    },
    {
    key: "PDM-7114",
    added: true
    }
    ],

    1466160360000: [
    {
    key: "PDM-12865",
    timeC: {
    oldEstimate: 0,
    newEstimate: 0,
    changeDate: 1466160360000
    },
    column: {
    notDone: false,
    done: true,
    newStatus: "6"
    }
    }
    ],
    1466160537000: [
    {
    key: "PDM-12865",
    timeC: {
    oldEstimate: 0,
    newEstimate: 0,
    changeDate: 1466160537000
    }
    }
    ],


    startTime: 1465387620000,
    endTime: 1466185620000,
    completeTime: 1466434989792,
    now: 1466442135123,
    statisticField: {
    typeId: "field",
    fieldId: "timeestimate",
    id: "field_timeestimate",
    name: "Remaining Time Estimate",
    isValid: true,
    isEnabled: true,
    renderer: "duration"
    },
    issueToParentKeys: {
    PDM-12815: null,
    PDM-12417: null,
    PDM-12865: "PDM-10065",
    PDM-12864: "PDM-10065",
    PDM-6730: null,
    PDM-12861: "PDM-12420",
    PDM-12420: null,
    PDM-12860: "PDM-12420"
    },
    workRateData: {
    timezone: "Asia/Singapore",
    rates: [

    {
    start: 1466208000000,
    end: 1466380800000,
    rate: 0
    },
    {
    start: 1466380800000,
    end: 1466434989792,
    rate: 1
    }
    ]
    },
    openCloseChanges: {
    1466434989881: [
    {
    userDisplayNameHtml: "<a class="user-hover" rel="xxxx" id="_xxxx" href="/secure/ViewProfile.jspa?name=xxxx">xxxx</a>",
    operation: "CLOSE"
    }
    ]
    },
    lastUserWhoClosedHtml: "<a class="user-hover" rel="xxxx" id="_xxxx" href="/secure/ViewProfile.jspa?name=xxxx">xxxxxx</a>"
    }

和我的PHP代码（跳过卷曲部分）是：

<?php

$result=curl_exec($ch);

curl_close($ch);

$res= (array)json_decode($result);

$res= json_encode($res,true);
echo $res;


?>