$_callback
$_callback : string|array
PHP callback to invoke
Publish-Subscribe handler: unique handle subscribed to a given topic.
__construct(string $topic, string|object $context, string|null $handler = null)
Constructor
string | $topic | Topic to which handle is subscribed |
string|object | $context | Function name, class name, or object instance |
string|null | $handler | Method name, if $context is a class or object |