sabre/event must be intalled using composer.
You will need a recent version of PHP. sabre/event 5 requires PHP 7, but sabr/event 3 is still actively maintained and works on PHP 5.5 and up.
Installing with composer
If composer is not yet on your system, follow the instructions on getcomposer.org to install it.
From your project directory, the easiest is then to simply call:
composer require sabre/event ^5
This rule ensures that you install the latest event package in the 5.x range of packages.
After running this, sabre/event should be installed, and you can load it in by including the autoloader:
The sabre/event source can be found on GitHub.