rector/docs/HowUseWithOldPhp.md

How to use on PHP < 7.1 or Incompatible Composer Dependencies

You must have an isolated environment with PHP 7.1 (for example in a Docker container). When you have it then run following command:

composer create-project rector/rector path-to-rector

You will be able to run Rector like this:

path-to-rector/bin/rector <arguments>

Now you can read more in README.