PCNTL is a PHP extension that cannot be loaded like other more convenient extension. This is because of the great power it gives to PHP. PCNTL allows you to create process directly into the server side machine. While this may seen as something you would want, it also gives the power to PHP to mess things up really bad.
To enable (By default is not enable) this extension PCNTL has to be compiled into PHP. For this situation arch provides several tools which users who use AUR services may be familiar.
Tools: makepkg abs