The developer who created a platform my company uses is no longer working for us and I don\'t know how I can retrieve the passwords from a custom PHP application
Wh
With luck, if the original developer was any good, you will not be able to get the plain text out. I say "luck" otherwise you probably have an insecure system.
For the admin passwords, as you have the code, you should be able to create hashed passwords from a known plain text such that you can take control of the application. Follow the algorithm used by the original developer.
If they were not salted and hashed, then make sure you do apply this as 'best practice'