How to save nested json output in csv file in php?

I have generated json output using API and now I have to save this output in csv file in below structure- The column names are litle bit different from the actual name. Some records are coming as null.

Example:

json_data =

"data": [
        {
            "id": 1000001,
            "version_modified_date__v": "2022-03-01T08:23:04.000Z",
            "doc_roles__sysr": {
                "responseDetails": {
                    "pagesize": 250,
                    "pageoffset": 0,
                    "size": 22,
                    "total": 22
                },
                "data": [
                    {
                        "role_name__sys": "emea_project_owner__c",
                        "user__sys": 3566601,
                        "group__sys": null
                    },
                    {
                        "role_name__sys": "emea_submitter__c",
                        "user__sys": 3566601,
                        "group__sys": null
                    }
                ]
            }
        },
        {
            "id": 1000002,
            "version_modified_date__v": "2021-02-24T17:58:05.000Z",
            "doc_roles__sysr": {
                "responseDetails": {
                    "pagesize": 250,
                    "pageoffset": 0,
                    "size": 9,
                    "total": 9
                },
                "data": [
                    {
                        "role_name__sys": "owner__c",
                        "user__sys": 8272503,
                        "group__sys": null
                    },
                    {
                        "role_name__sys": "reference_specialist__c",
                        "user__sys": null,
                        "group__sys": 1461227836262
                    }
                ]
            }
        }
    ]

I want in CSV file in below format -

id       version_modified_date     role name               user id     group
1000001  2022-03-01T08:23:04.000Z  emea_project_owner__c   3566601
1000001  2022-03-01T08:23:04.000Z  emea_submitter__c       3566601
1000002  2021-02-24T17:58:05.000Z  owner__c                8272503
1000002  2021-02-24T17:58:05.000Z  reference_specialist__c             1461227836262

I tried below code but didn't get any luck. Please help me to correct this code-

                        $content = json_decode(json_encode($content), true);
                        $fp = fopen('doc_roles__sys.csv', 'w');
                        foreach ($content['data'] as $item) {
                            $output['id'] = $item['id'];
                            $output['version_modified_date'] = $item['version_modified_date__v'];
                            
                            $output['role name'] = $item['data'][0]['role_name__sys'];
                            $output['user id'] = $item['data'][0]['user__sys'];
                            $output['group'] = $item['data'][0]['group__sys'];
                            //$output['document_id'] = $item['data'][0]['document_id'];
                            foreach ($output as $line) {
                                 fputcsv($fp, $item);
                            }
                        }   

                        echo $output;

                        //$fp = fopen('doc_roles__sys.csv', 'a');
                        //foreach ($output as $line) {
                        //  fputcsv($fp, $item);
                        //}
                        fclose($fp);

The output I am getting from above code is -

1077079,Array
1077079,Array
1077079,Array
1077079,Array
1077079,Array