PayPal\Auth\Oauth\OAuthServer::get_signature_method PHP Method

    private function get_signature_method($request)
    {
        $signature_method = $request instanceof OAuthRequest ? $request->get_parameter("oauth_signature_method") : null;
        if (!$signature_method) {
            // According to chapter 7 ("Accessing Protected Ressources") the signature-method
            // parameter is required, and we can't just fallback to PLAINTEXT
            throw new OAuthException('No signature method parameter. This parameter is required');
        }
        if (!in_array($signature_method, array_keys($this->signature_methods))) {
            throw new OAuthException("Signature method '{$signature_method}' not supported " . "try one of the following: " . implode(", ", array_keys($this->signature_methods)));
        }
        return $this->signature_methods[$signature_method];
    }