protected function computeCanonicalizedHeaders($headers)
    {
        $canonicalizedHeaders = array();
        $normalizedHeaders    = array();
        $validPrefix          =  Resources::X_MS_HEADER_PREFIX;

        if (is_null($normalizedHeaders)) {
            return $canonicalizedHeaders;
        }

        foreach ($headers as $header => $value) {
            // Convert header to lower case.
            $header = strtolower($header);

            // Retrieve all headers for the resource that begin with x-ms-,
            // including the x-ms-date header.
            if (Utilities::startsWith($header, $validPrefix)) {
                // Unfold the string by replacing any breaking white space
                // (meaning what splits the headers, which is \r\n) with a single
                // space.
                $value = str_replace("\r\n", ' ', $value);

                // Trim any white space around the colon in the header.
                $value  = ltrim($value);
                $header = rtrim($header);

                $normalizedHeaders[$header] = $value;
            }
        }

        // Sort the headers lexicographically by header name, in ascending order.
        // Note that each header may appear only once in the string.
        ksort($normalizedHeaders);

        foreach ($normalizedHeaders as $key => $value) {
            $canonicalizedHeaders[] = $key . ':' . $value;
        }

        return $canonicalizedHeaders;
    }