PayPal\Auth\Oauth\OAuthSignatureMethodHmacSha1::build_signature PHP Method

    public function build_signature($request, $consumer, $token)
    {
        $base_string = $request->get_signature_base_string();
        $base_string = preg_replace_callback("/(%[A-Za-z0-9]{2})/", array($this, "replace_callback"), $base_string);
        //convert base string to lowercase
        $request->base_string = $base_string;
        $key_parts = array($consumer->secret, $token ? $token->secret : "");
        $key_parts = OAuthUtil::urlencode_rfc3986($key_parts);
        $key = implode('&', $key_parts);
        $key = preg_replace_callback("/(%[A-Za-z0-9]{2})/", array($this, "replace_callback"), $key);
        //convert to lowercase
        return base64_encode(hash_hmac('sha1', $base_string, $key, true));
    }