src/Composer/Command/DiagnoseCommand.php
private function checkHttpProxyFullUriRequestParam()
    {
        $result = $this->checkConnectivity();
        if ($result !== true) {
            return $result;
        }

        $url = 'http://repo.packagist.org/packages.json';
        try {
            $this->rfs->getContents('packagist.org', $url, false);
        } catch (TransportException $e) {
            try {
                $this->rfs->getContents('packagist.org', $url, false, array('http' => array('request_fulluri' => false)));
            } catch (TransportException $e) {
                return 'Unable to assess the situation, maybe packagist.org is down ('.$e->getMessage().')';
            }

            return 'It seems there is a problem with your proxy server, try setting the "HTTP_PROXY_REQUEST_FULLURI" and "HTTPS_PROXY_REQUEST_FULLURI" environment variables to "false"';
        }

        return true;
    }
private function checkHttpProxyFullUriRequestParam()
    {
        $result = $this->checkConnectivity();
        if ($result !== true) {
            return $result;
        }

        $url = 'http://repo.packagist.org/packages.json';
        try {
            $this->rfs->getContents('packagist.org', $url, false);
        } catch (TransportException $e) {
            try {
                $this->rfs->getContents('packagist.org', $url, false, array('http' => array('request_fulluri' => false)));
            } catch (TransportException $e) {
                return 'Unable to assess the situation, maybe packagist.org is down ('.$e->getMessage().')';
            }

            return 'It seems there is a problem with your proxy server, try setting the "HTTP_PROXY_REQUEST_FULLURI" and "HTTPS_PROXY_REQUEST_FULLURI" environment variables to "false"';
        }

        return true;
    }
Due to various proxy servers configurations, some servers can't handle non-standard HTTP "http_proxy_request_fulluri" parameter,
and will return error 500/501 (as not implemented), see discussion @ https://github.com/composer/composer/pull/1825.
This method will test, if you need to disable this parameter via setting extra environment variable in your system.

@return bool|string