The Fortrabbit blog has a post that want to help you store your database password securely and away from prying attacker eyes. While they use the example of a a database password, credentials for just about any other service could be protected the same way.

How do you protect your access data? Your sensitive secrets, basically anything your PHP application uses to authenticate or authorize with other services such as databases, caches, cloud storages, image resize services, transactional mail providers. All of them. Where do you put this — easily accessible while in development and secure for production?

They start by pointing out a few places where they should not be stored: in your code, in a version control system or in an environment variable (plain text). Instead, they suggest using a combination of a secret key that's configured in the application and encrypted versions of the values in environment variables. Some code is included showing how to set this up in a Laravel-based application, but the principle can be applied independent of the framework too with some other simple tools. They end the post with some links to other articles including a "considered harmful" piece reinforcing their methods.