I was playing with something like this a long time ago back when I was excited about NodeJS. I hooked up libuv to PHP and got https://github.com/dtjm/php-async which seemed to work in simple experiments.
php-tokio plugs into revolt, which in turn allows it to coexist with any async library based on or compatible with revolt (i.e. the entire amp ecosystem, PSL, reactphp, etc...).