Presumably, you pick/choose which would use swoole or be delivered on php-fpm maybe run both with nginx proxying swoole, then your old code would just run sync

Presumably, you pick/choose which would use swoole or be delivered on php-fpm maybe run both with nginx proxying swoole, then your old code would just run sync and newer could run async if required --maybe for things like chat/websockets and not for just static pages.