Auth0\SDK\API\Authentication::oauth_token PHP Method

    public function oauth_token($client_id, $client_secret, $grant_type = 'client_credentials', $code = null, $audience = null, $scope = null)
    {
        $data = ['client_id' => $client_id, 'client_secret' => $client_secret, 'grant_type' => $grant_type];
        if ($audience !== null) {
            $data['audience'] = $audience;
        }
        if ($scope !== null) {
            $data['scope'] = $scope;
        }
        if ($code !== null) {
            $data['code'] = $code;
        }
        return $this->apiClient->post()->oauth()->token()->withHeader(new ContentType('application/json'))->withBody(json_encode($data))->call();
    }