I actually mentioned Haskell in the post because coworkers have lamented not having its type system, which would immediately find problems at compile-time that we otherwise have a very very hard time thoroughly covering with tests.
The really really hard part is still checking that a given page "isn't broken"—something that's easy for a human to spot but hard for a computer—for all possible combinations of state. But very strict typing would at least make the output less likely to break.
What's so insanely monumental about writing unit, functional (i.e. like these http://symfony.com/doc/current/book/testing.html#functional-...) and Selenium tests to cover everything before committing new changes? It's just like any other code - test the main condition and edge cases and everything will be running smoothly next time you come back to that feature.