PHP cURL、POST JSON

我需要发布以下 JSON 代码，但由于某种原因它不起作用。下面是我的代码。

$fieldString = "395609399";
//the curl request processor
function processCurlJsonrequest($URL, $fieldString) { //Initiate cURL request and send back the result
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_HTTPHEADERS, array('Content-Type: application/json'));
    curl_setopt($ch, CURLOPT_URL, $URL);
    curl_setopt($ch, CURLOPT_USERAGENT, $this->_agent);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    curl_setopt($ch, CURLOPT_COOKIEFILE, $this->_cookie_file_path);
    curl_setopt($ch, CURLOPT_COOKIEJAR, $this->_cookie_file_path);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, TRUE);
    curl_setopt($ch, CURLOPT_VERBOSE, TRUE);
    if ($fieldCount) { // in case of post fields present then pass it on
        curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode("{categoryId: $fieldString}"));
        curl_setopt($ch, CURLOPT_POST, 1);
    }
    $resulta = curl_exec($ch);
    if (curl_errno($ch)) {
        print curl_error($ch);
    } else {
        curl_close($ch);
    }
    return $resulta;
}

这是调用 cURL 请求的函数：

function get_cat($categoryId, $URL) {
    $fields = array(
        "categoryId" => $categoryId
    );
    $fields_string = $fields;
    return $this->processCurlJsonrequest($URL, $fields_string);
}

问题所在是：

curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode("{categoryId: $fieldString}"));

CURLOPT_POSTFIELDS 将接受参数数组或 URL 编码的参数字符串：

curl_setopt($ch, CURLOPT_POSTFIELDS, array('json'=>json_encode($stuff)));
curl_setopt($ch, CURLOPT_POSTFIELDS, 'json='.urlencode(json_encode($stuff)));

Where json将是 POST 字段的名称（即：将导致$_POST['json']是可访问的）。