* Clarify what a built-in type is.
* Add PHP 8 equivalent of ReflectionParameter::isArray().
* Leave note on ReflectionParameter::getClass() for what to use instead.
* Clarify order of the array in getParameters().
* Add example to aid working with union types.
* Add updated equivalent of isCallable().
We also document the new `$filter` parameter for the `ReflectionClass`
methods `::getConstants()` and `::getReflectionConstants()` and the
respective constants.
Co-authored-by: Christoph M. Becker <cmbecker69@gmx.de>
Closes GH-298.
