About undefined vs. NULL:

> Not a warning—a FATAL error occurs if you try to access an uninitialized property. This comes up a lot in cases where you try to deserialize data into a PHP object. If a field's data isn't present you might not initialize the property at all.

I don't think that is an issue, except in interpretet type-unsafe languages it is harder to anticipate when writing code whether that value is NULL or undefined/uninitialized. E.g. it is basically the same in C#, but here the compiler warns you that the value is not initialized and forbids some actions (like reading the value of it).