Event callbacks

If a callback is registered for an event, it will be called when the event becomes active. To associate a callback with event one can pass a callable to either Event::__construct(), or Event::set(), or one of the factory methods like Event::timer().

An event callback should match the following prototype:

function callback( mixed $fd = null , int $what = ?, mixed $arg = null ): void

fd: The file descriptor, stream resource or socket associated with the event. For signal event fd is equal to the signal number.
what: Bit mask of all events triggered.
arg: User custom data.

Event::timer() expects the callback to match the following prototype:

function callback( mixed $arg = null ): void

arg: User custom data.

Event::signal() expects the callback to match the following prototype:

function callback( int $signum = ?, mixed $arg = null ): void

signum: The number of the triggered signal(e.g. SIGTERM ).
arg: User custom data.