Selenium server is useful especially when you need to execute multiple tests at once or your tests are run in different browsers - like on your CI server.

Selenium server receives commands from php-webdriver and starts new sessions using browser drivers. It acts like hub distributing the commands among one or multiple nodes.

To run the standalone server, download selenium-server-4.X.Y.jar. The latest version is available on the official Selenium download page or in GitHub releases.

Run from command line terminal (replace # with server version).

java -jar selenium-server-#.jar

You may need to provide path to chromedriver/geckodriver binary (if they are not placed in system PATH directory):

# Chromedriver:
java -Dwebdriver.chrome.driver="/opt/chromium-browser/chromedriver" -jar vendor/bin/selenium-server-#.jar
# Geckodriver:
java -Dwebdriver.gecko.driver="/home/john/bin/geckodriver" -jar vendor/bin/selenium-server-#.jar

These options could be combined and you can define paths for both chromedriver and geckodriver.

If you want to distribute browser sessions among multiple servers ("grid mode" - one Selenium hub and multiple Selenium nodes) please refer to the Grid documentation.

Selenium server could also be started inside Docker container with browser and browser drivers built in - see docker-selenium project.

After your selenium server is up and running (either by running the jar file or in Docker), you can start to initiate the browser of your choice from your script:

$host = 'https://localhost:4444'; // for selenium server version 4.x
$desiredCapabilities = DesiredCapabilities::chrome(); // or DesiredCapabilities::firefox(); etcv.
$driver = RemoteWebDriver::create($host, $desiredCapabilities);

For more information see official grid documentation.