Example for an HTTP and SSL proxy:

$seleniumServerUrl = 'http://localhost:4444';
$proxyUrl = '127.0.0.1:2043';

$capabilities = new DesiredCapabilities(
    [
        WebDriverCapabilityType::BROWSER_NAME => 'firefox',
        WebDriverCapabilityType::PROXY => [
            'proxyType' => 'manual',
            'httpProxy' => $proxyUrl,
            'sslProxy' => $proxyUrl,
            'noProxy' => ['google.com'],
    ],
]);
$driver = RemoteWebDriver::create($seleniumServerUrl, $capabilities);

See W3C WebDriver specification for more proxy configuration options.

Using proxy with credentials is currently not implemented in neither Chrome or Firefox, see related issues:

• Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=615947
• Firefox https://github.com/mozilla/geckodriver/issues/1872, https://bugzilla.mozilla.org/show_bug.cgi?id=1395886

As a workaround, it is recommended to put you own local proxy (for example mitmproxy) in front of the remote authenticated proxy. You will then pass URL of the local proxy (eg. localhost:8080) as part of capabilities (see example above), and configure this custom proxy to forward requests to the remote proxy with authentication.

For example to start local mitmproxy which will forward requests to remote proxy running on https://hostname:port with credentials username:password use:

mitmproxy --mode upstream:https://hostname:port --upstream-auth username:password

This will start proxy on localhost:8080 (use $proxyUrl = 'localhost:8080' as per example above).