If you want to install Midrub on PHP 7.1+ you have to upgrade the CodeIgniter.
How to upgrade the CodeIgniter?
First, delete the directory system from the Midrub directory.
Then, download Codeigniter 3.1.4 from here https://github.com/bcit-ci/CodeIgniter/archive/3.1.4.zip
Extract only the directory system and copy in the directory Midrub.
After that, Midrub will be compatible with PHP 7.1+.