private function _generateSignatureKey()
    {
        $date_h = hash_hmac('sha256', $this->_date, 'AWS4' . $this->_aws_secret, true);
        $region_h = hash_hmac('sha256', $this->_region, $date_h, true);
        $service_h = hash_hmac('sha256', self::SERVICE, $region_h, true);
        $signing_h = hash_hmac('sha256', 'aws4_request', $service_h, true);

        return $signing_h;
    }
private function _generateSignatureKey()
    {
        $date_h = hash_hmac('sha256', $this->_date, 'AWS4' . $this->_aws_secret, true);
        $region_h = hash_hmac('sha256', $this->_region, $date_h, true);
        $service_h = hash_hmac('sha256', self::SERVICE, $region_h, true);
        $signing_h = hash_hmac('sha256', 'aws4_request', $service_h, true);

        return $signing_h;
    }
private function _generateSignatureKey ( ) { $ date_h = hash_hmac ( 'sha256' , $ this -> _date , 'AWS4' . $ this -> _aws_secret , true ) ; $ region_h = hash_hmac ( 'sha256' , $ this -> _region , $ date_h , true ) ; $ service_h = hash_hmac ( 'sha256' , self :: SERVICE , $ region_h , true ) ; $ signing_h = hash_hmac ( 'sha256' , 'aws4_request' , $ service_h , true ) ; return $ signing_h ; }
Create and returns binary hmac sha256

@return hmac sha256.
Create and returns binary hmac sha256