PHP cURL, POST JSON

J'ai besoin de mettre les éléments suivants du code JSON, mais pour quelque raison il ne fonctionne pas. Ci-dessous le code que j'ai.

$fieldString = "395609399";
//the curl request processor
function processCurlJsonrequest($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;
}

Ici, est la fonction qui appelle à la demande cURL:

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

source d'informationauteur Michael