Hello Matthew,
To install with Composer this package, you need to follow these steps:
1. Make sure you logged in to the site
2. Click on the Download tab of this package:
phpclasses.org/package/12890-PHP-Cr
...
3. Click on the Install with Composer button
4. The page will show the lines that you need to add to your composer.json file and the user name and password to access the Composer repository of the PHP Classes site using your site account.
Can you try following these steps and let us know if you can install the package using Composer?