sabre/uri must be installed using composer.
You will need a recent version of PHP. sabre/uri 2.x requires PHP 7, but 1.x has an identical api and supports 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/uri ^2
This rule ensures that you install the latest stable uri package, and you'll get any future updates (which never break backwards compatibility).
After running this, sabre/uri should be installed, and you can load it in by including the autoloader:
The sabre/uri source can be found on GitHub.