Question about docker images

I’m super new to CI, this won’t be my last time asking questions.

so I’m setting up my config file and I see I have a lot of options for PHP images:

https://hub.docker.com/r/circleci/php/tags/

I’m lost as to which image I should grab, I just know I need PHP 7.1

what software is preloaded on these images? is it just a clean install of linux?

Thanks