At Small Shops United, we use MadMimi for our email list software. I took a look at the PHP libraries that were available and found them extremely out of date, lacking, and just plain not working in certain areas.
I had the need to integrate with their services, so I decided to create an updated open source PHP library. You can find that here: github.com/aaronsaray/madmimi-api-php. I’m currently using this in production and it’s performing great.
Next step - get MadMimi to add it to their support page. Here’s hoping! (At least it’s in packagist for composer install, though.)