Here you find the DSL language definition for your phrases you can use in .feature files:
features/bootstrap/FeatureContext.php inherits from
vendor/behat/mink-extension/Behat/MinkExtension/Context/MinkContext.php
With Homebrew that gets really easy:
brew install selenium-server-standalone
before running bin/behat
start up the selenium server:
java -jar /usr/local/opt/selenium-server-standalone/selenium-server-standalone-2.35.0.jar -p 4444