I wanted to answer this. If you are using pretty much any PHP extension. You need to complete the following

1. https://devcenter.heroku.com/articles/php-support#extensions - Look at this link and find your extension. (If its not on this list you cant use it)
2. Go into your composer.json file and update your require section. As an example “require”: { “php”: “^7.2”, “ext-json”: “”, “ext-zip”: “”, “ext-imagick”: “”, “ext-gd”: “”,
3. Run Composer Update to update your lock file
4. Upload your changes to github or whatever service is currently supported by DO
5. Watch your changes deploy.