I was curious about why setting `$this->countryCode` inside the setter for `countryCode` didn't result in infinite recursion. Turns out this is spelled out in the RFC, but not in the docs:
When a hook is called, inside that hook $this->[propertyName] will refer to the “unfiltered” value of the property, called the “backing value.” When accessed from anywhere else, $this->[propertyName] calls will go through the relevant hook. This is true for all hooks on the same property. This includes, for example, writing to a property from the get hook; that will write to the backing value, bypassing the set hook.
A normal property has a stored “backing value” that is part of the object, and part of the memory layout of the class. However, if a property has at least one hook, and none of them make use of $this->[propertyName], then no backing value will be created and there will be no data stored in the object automatically (just as if there were no property, just methods). Such properties are known as “virtual properties,” as they have no materialized stored value.
Be aware, the detection logic works on $this->[propertyName] directly at compile time, not on dynamic forms of it like $prop = 'beep'; $this->$prop. That will not trigger a backing value.
Feels like too much magic to me that a property access can mean different things depending on context, but I'm not a PHP user, so I don't get a vote.