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Event callbacks

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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.

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:

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:

callback(mixed $arg = null): void

arg: User custom data.

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

callback([int $signum], mixed $arg = null): void

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

Source: reference/event/event.callbacks.xml · from the official PHP manual (php/doc-en)