Underscore method prefix

Question

I\'ve been examining the code of CodeIgniter and CakePHP and I noticed that some of the methods in their classes are prefixed with an underscore _ or a double u

Answer 1

They are probably magic methods. There is a number of those methods that serve a specific purpose (object constructor, object destructor, getter, setter...)

PHP reserves the __ prefix in function names for those magical functions. It's recommended not to define functions with that prefix for any other purpose.

Update: Both frameworks seem to use the __ prefix for their own purposes as well. See @Gordon's answer.