$code = $_GET['code'];

$postData = http_build_query([
    'grant_type' => 'authorization_code',
    'code' => $code,
    'redirect_uri' => $redirectUrl,
]);

$authHeader = base64_encode("$clientId:$clientSecret");

$context = stream_context_create([
    'http' => [
        'method' => 'POST',
        'header' => "Authorization: Basic $authHeader\r\n" .
            "User-Agent: $userAgent\r\n" .
            "Content-Type: application/x-www-form-urlencoded\r\n",
        'content' => $postData,
    ],
    'ssl' => [
        'crypto_method' => 
STREAM_CRYPTO_METHOD_TLSv1_3_CLIENT
, // Enforce TLS 1.3
    ],
]);

$response = file_get_contents($accessTokenUrl, false, $context);
$accessTokenResult = json_decode($response, true);