private function generateSasToken($uri)
    {
        $targetUri = strtolower(rawurlencode(strtolower($uri)));
        $expires = time();
        $expiresInMins = 60;
        $expires = $expires + $expiresInMins * 60;
        $toSign = $targetUri."\n".$expires;
        $signature = rawurlencode(base64_encode(hash_hmac('sha256', $toSign, $this->sasKeyValue, true)));
        $token = 'SharedAccessSignature sr='.$targetUri.'&sig='.$signature.'&se='.$expires.'&skn='.$this->sasKeyName;

        return $token;
    }