Aside from a HTTP component ReactPHP also has a HTTP Client component that lets your send out HTTP requests. It is incredibly handy when you need to communicate with for example elasticsearch's REST API, AWS platform through their SDK or the RIPE Atlas API.
The composer cache greatly speeds up your composer part of the build by only going to Github for new downloads. When combined with test lowest, current, and highest possible on Travis you only reach out to Github for new versions. Most likely to happen during the
highest possible set of builds, but also when you've updated
composer.*. This normally isn't an issue unless you hit Github's rate limit. And since composer is running on a 'public' travis box with a 'public' IP address that has been use by many builds before it there is a very very high chance it already hit the 60 requests per hour limit.