These types declarations can only be used within classes.
self
The value must be an instanceof the same class as the one
in which the type declaration is used.
parent
The value must be an instanceof a parent of the class
in which the type declaration is used.
static
static is a return-only type which requires that the
value returned must be an instanceof the same class as the one
the method is called in.
Available as of PHP 8.0.0.