Despite PHP not being a strongly-typed language, I am a great proponent of keeping code as strongly typed as possible. Therefore, rather than returning multiple types like many core PHP functions (for example strpos, which returns an integer or FALSE if the substring could not be found), we should return NULL to indicate that no appropriate return value was available.

I also strongly agree that NULL should never be used in error cases. Throwing an exception (or a range of different exceptions depending on the error state) allows the function caller to handle any error cases as required, ensuring the application remains in a stable state.

Unfortunately in PHP returning nothing and returning NULL are exactly the same, as opposed to languages such as JavaScript where NULL (the intentional absence of a value)and UNDEFINED (the unintentional absence of a value) are distinct types, however, as you say, it is still good practice to include a return statement in a function even if it doesn’t return anything.