Sammy Kaye Powers, the host of the PHP Roundtable podcast, wrote a blogpost on how to compile PHP from source including unreleased RFC's. After following this tutorial you'll be able to play with the [typed properties] that are currently under discussion.

We'll be discovering and testing a completely unreleased feature of php-src from an RFC that's still under discussion. If you've ever wanted to be ahead of the curve of PHP features or you've just wanted to contribute back to PHP internals, testing an unreleased feature from an RFC is a fun and educational way to do so.

https://www.sammyk.me/how-to-compile-an-unreleased-rfc-feature-for-php-source-php-internals