Coroutine style API became the mainstream in the Swoole core since version 4.3.0.
ext-async depends on swoole, it is a extension of swoole, including the async callback style API.
You have to install Swoole extension before installing this extenion.
Async clients and API are moved to a separate PHP extension swoole_async since version 4.3.0, install swoole_async:
git clone https://github.com/swoole/async-ext.git
cd async-src
phpize
./confiugre
make -j 4
sudo make installEnable it by adding a new line extension=swoole_async.so to php.ini.