[php-src] Issue #11411: nullable bool

Issue: https://github.com/php/php-src/issues/11411 Author: wdmeest ### Description It would be nice to have a nullable bool type (like in other languages). Now the bool type is true or false, but in some cases you don't know the status of the bool value yet. Example: You have a model 'Project' with some basic properties (title, description, ....). After a certain period of time you want to add a new bool property "$has_certain_data". You want the user to explicitly set this value on true or false, meaning all the current data should have the value null, as the value is not yet known.