php取出json返回null

代码如下:

<?php
$url = 'http://music.163.com/api/song/detail/?id='.$_GET['id'].'&ids=%5B'.$_GET['id'].'%5D&csrf_token=';
$json = file_get_contents($url);
$obj = json_decode($json);
$array = array(
    "name" => $obj->songs[0]->bMusic->name,
    "artist" => $obj->songs[0]->artists[0]->name,
    "url" => $obj->songs[0]->mp3Url
);
echo json_encode($array);
?>

json文件大致是这样:

{
    "songs": [
        {
            "starred": false,
            "popularity": 95,
            "starredNum": 0,
            "playedNum": 0,
            "dayPlays": 0,
            "hearTime": 0,
            "mp3Url": "http://m2.music.126.net/2b980AHtVx18bJ_Z9MhKXA==/2830142929914115.mp3",
            "rtUrls": [],
            "status": 1,
            "crbt": null,
            "bMusic": {
                "volumeDelta": 0.095421,
                "playTime": 200000,
                "bitrate": 96000,
                "dfsId": 2830142929914115,
                "sr": 44100,
                "name": "Set Fire To The Rain",
                "id": 24667967,
                "size": 2436177,
                "extension": "mp3"
            },
            "rtUrl": null,
            "position": 2,
            "duration": 200000,
            "alias": [],
            "hMusic": {
                "volumeDelta": -0.05,
                "playTime": 200000,
                "bitrate": 320000,
                "dfsId": 2830142929914113,
                "sr": 44100,
                "name": "Set Fire To The Rain",
                "id": 24667965,
                "size": 8056892,
                "extension": "mp3"
            },
            "mMusic": {
                "volumeDelta": 0.41,
                "playTime": 200000,
                "bitrate": 160000,
                "dfsId": 2830142929914114,
                "sr": 44100,
                "name": "Set Fire To The Rain",
                "id": 24667966,
                "size": 4042394,
                "extension": "mp3"
            },
            "lMusic": {
                "volumeDelta": 0.095421,
                "playTime": 200000,
                "bitrate": 96000,
                "dfsId": 2830142929914115,
                "sr": 44100,
                "name": "Set Fire To The Rain",
                "id": 24667967,
                "size": 2436177,
                "extension": "mp3"
            },
            "album": {
                "songs": [],
                "paid": false,
                "onSale": false,
                "status": 1,
                "tags": "",
                "blurPicUrl": "http://p4.music.126.net/J32cgj_Dq-4Wpfs4U9sjwA==/2539871860943087.jpg",
                "companyId": 0,
                "pic": 2539871860943087,
                "description": "",
                "subType": null,
                "alias": [],
                "picId": 2539871860943087,
                "briefDesc": "",
                "artist": {
                    "img1v1Id": 0,
                    "alias": [],
                    "picId": 0,
                    "briefDesc": "",
                    "picUrl": "http://p4.music.126.net/6y-UleORITEDbvrOLV0Q8A==/5639395138885805.jpg",
                    "albumSize": 0,
                    "img1v1Url": "http://p3.music.126.net/6y-UleORITEDbvrOLV0Q8A==/5639395138885805.jpg",
                    "trans": "",
                    "musicSize": 0,
                    "name": "",
                    "id": 0
                },
                "picUrl": "http://p4.music.126.net/J32cgj_Dq-4Wpfs4U9sjwA==/2539871860943087.jpg",
                "commentThreadId": "R_AL_3_1983765",
                "artists": [
                    {
                        "img1v1Id": 0,
                        "alias": [],
                        "picId": 0,
                        "briefDesc": "",
                        "picUrl": "http://p4.music.126.net/6y-UleORITEDbvrOLV0Q8A==/5639395138885805.jpg",
                        "albumSize": 0,
                        "img1v1Url": "http://p3.music.126.net/6y-UleORITEDbvrOLV0Q8A==/5639395138885805.jpg",
                        "trans": "",
                        "musicSize": 0,
                        "name": "Piano Tribute Players",
                        "id": 99094
                    }
                ],
                "copyrightId": 0,
                "publishTime": 1321804800000,
                "company": "CC Entertainment",
                "name": "Piano Tribute to Adele",
                "id": 1983765,
                "type": "专辑",
                "size": 14
            },
            "commentThreadId": "R_SO_4_21515271",
            "artists": [
                {
                    "img1v1Id": 0,
                    "alias": [],
                    "picId": 0,
                    "briefDesc": "",
                    "picUrl": "http://p3.music.126.net/6y-UleORITEDbvrOLV0Q8A==/5639395138885805.jpg",
                    "albumSize": 0,
                    "img1v1Url": "http://p4.music.126.net/6y-UleORITEDbvrOLV0Q8A==/5639395138885805.jpg",
                    "trans": "",
                    "musicSize": 0,
                    "name": "Piano Tribute Players",
                    "id": 99094
                }
            ],
            "copyrightId": 0,
            "score": 95,
            "fee": 0,
            "mvid": 0,
            "ftype": 0,
            "rtype": 0,
            "rurl": null,
            "copyFrom": "",
            "audition": null,
            "ringtone": "",
            "disc": "",
            "no": 2,
            "name": "Set Fire To The Rain",
            "id": 21515271
        }
    ],
    "equalizers": {},
    "code": 200
}

上传到主机页面输出结果:

Notice: Trying to get property of non-object in on line 6 {"name":null,"artist":"Piano Tribute Players","url":null}

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