如何在PHP中获取JSON中的所有密钥？

How do I get all of the keys and loop through them, echoing each one, from a JSON object array?

Here is my code:

/* SAMPLE JSON THAT IS SENT
$mystring = '{
    "display_name": "Silverware",
    "fields": [
        {
            "field_name": "Age",
            "sort_order": 1,
            "required": 0,
            "view_type": "text",
            "description": "",
            "multi_value": 0,
            "char_count": 255
        },
        {
            "field_name": "Brand",
            "sort_order": 2,
            "required": 0,
            "view_type": "multiselect",
            "description": "",
            "multi_value": 1,
            "char_count": 255
        }
    ]
}';
*/


$json = json_decode($HTTP_RAW_POST_DATA);
$arr = $json->{'fields'};

// This is how I print a specific value
//print $arr[0]->{'field_name'};

Adding the following isn't working for me:

foreach ($arr as $k => $v) {
    echo $k', ';
}

Use true for the second parameter of json_decode().

foreach ($j['fields'] as $field) {
  $keys = array_keys($field);
  foreach ($keys as $key) {
    echo $key;
  }
}