I thought I was being as clear as possible.
When I upgrade the PHP version. I am no longer able to log in. It displays, invalid username or password.
Flip it back to the previous version of PHP and I can log in with the same username and password.
The only thing that changes is the system is the PHP version.
That is why I hypothesized that it has to be the keys. We are not passing clear text from the database to my knowledge. The password stored in the database needs to be decrypted and compared with the password entered in the form (this is my theory).
What else could be going wrong? Any suggestions are welcomed.