src/HTTP/Request2/Adapter/Socket.php
protected function shouldUseProxyDigestAuth(HTTP_Request2_Response $response)
    {
        if (407 != $response->getStatus() || !$this->request->getConfig('proxy_user')) {
            return false;
        }
        if (!($challenge = $this->parseDigestChallenge($response->getHeader('proxy-authenticate')))) {
            return false;
        }

        $key = 'proxy://' . $this->request->getConfig('proxy_host') .
               ':' . $this->request->getConfig('proxy_port');

        if (!empty(self::$challenges[$key])
            && (empty($challenge['stale']) || strcasecmp('true', $challenge['stale']))
        ) {
            $ret = false;
        } else {
            $ret = true;
        }
        self::$challenges[$key] = $challenge;
        return $ret;
    }
protected function shouldUseProxyDigestAuth(HTTP_Request2_Response $response)
    {
        if (407 != $response->getStatus() || !$this->request->getConfig('proxy_user')) {
            return false;
        }
        if (!($challenge = $this->parseDigestChallenge($response->getHeader('proxy-authenticate')))) {
            return false;
        }

        $key = 'proxy://' . $this->request->getConfig('proxy_host') .
               ':' . $this->request->getConfig('proxy_port');

        if (!empty(self::$challenges[$key])
            && (empty($challenge['stale']) || strcasecmp('true', $challenge['stale']))
        ) {
            $ret = false;
        } else {
            $ret = true;
        }
        self::$challenges[$key] = $challenge;
        return $ret;
    }
Checks whether another request should be performed with proxy digest auth

Several conditions should be satisfied for it to return true:
- response status should be 407
- proxy auth credentials should be set in the request object
- response should contain Proxy-Authenticate header with digest challenge
- there is either no challenge stored for this proxy or new challenge
contains stale=true parameter (in other case we probably just failed
due to invalid username / password)

The method stores challenge values in $challenges static property

@param HTTP_Request2_Response $response response to check

@return   boolean whether another request should be performed
@throws   HTTP_Request2_Exception in case of unsupported challenge parameters