Converting JSON to CSV format using PHP

I am trying to convert a json file into csv format using a php script. The code is as follows:

if (empty($argv[1])) die("The json file name or URL is missed\n");
$jsonFilename = $argv[1];

$json = file_get_contents($jsonFilename);
$array = json_decode($json, true);
$f = fopen('output.csv', 'w');

$firstLineKeys = false;
foreach ($array as $line)
{
    if (empty($firstLineKeys))
    {
            $firstLineKeys = array_keys($line);
            fputcsv($f, $firstLineKeys);
            $firstLineKeys = array_flip($firstLineKeys);
    }

fputcsv($f, array_merge($firstLineKeys, $line));

}

This kind of works, but is only returning the outer variables of the JSON file, and am getting a "Array to string conversion" warning

The JSON data looks like this:

{"type":"NON_ATTRIBUTED","conversion":{,"value_1":"000000100355321","value_3":"XXXX","value_4":"12667","value_5":"6"},"stream_type":"COOKIE"}
{"type":"ATTRIBUTED","conversion":{,"value_1":"000000167865321","value_3":"YYYY","value_4":"12668","value_5":"0"},"stream_type":"COOKIE"}
{"type":"NON_ATTRIBUTED","conversion":{,"value_1":"000000134535321","value_3":"AAAA","value_4":"12669","value_5":"9"},"stream_type":"COOKIE"}
{"type":"NON_ATTRIBUTED","conversion":{,"value_1":"000000100357651","value_3":"WWWW","value_4":"12670","value_5":"2"},"stream_type":"COOKIE"}

The output I am getting is : type,conversion,stream_type NON_ATTRIBUTED,Array,COOKIE NON_ATTRIBUTED,Array,COOKIE

The output I am expecting is: type,conversion,value_1,value_3,value_4, value_5 ,stream_type NON_ATTRIBUTED,000000100355321, XXXX, 1267, 6, COOKIE ..

ANy help appreciated as this is very new to me