Php: How To Store System Configurations On A Database And Use Them As Constants?

I’d like to have constants in PHP which are retrieved from a database. Essentially, I’d like to create a page that can update this database table so the next time I load the site, it will use these newly updated constants which are again retrieved from the database.
I’m looking for things that can be changed directly by the admin without needing to go into the source code and manually changing the hardcoded constants file.