Usually only in an authors head when trying to flog you their latest "Better software quality with {hype-technology-of-the-week}" book.
As soon as you get into interactive web pages with lots of daily changing dynamic data in a dynamic environment, the best test is a second pair of eyes during development/code changes and than re-test the changed part on a dedicated test server manually after importing the latest database backup.
perl -e 'use MIME::Base64; print decode_base64("4pmsIE5ldmVyIGdvbm5hIGdpdmUgeW91IHVwCiAgTmV2ZXIgZ29ubmEgbGV0IHlvdSBkb3duLi4uIOKZqwo=");'