Better way to validate user input in cakephp
I would split up the code into objects, with good object oriented patterns. This should result in objects that have very distinct responsibilities.

The pattern I think you should primarily look into is the "Proxy" pattern (See "Design Patterns" book by "Gang of four".) It can be used to extract the code that ensures security, for example. It makes the security code independent from the domain logic / data access / user interface. 

It may mean refactoring a lot of code, but once done, you can then write thorough unit-tests for the security proxy in isolation, giving you a rock solid foundation.

Write tests, then write the testable code, (i.e. isolated) then refactor to the point where it's neat and tidy.