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A action can be defined in a separate file in Yaf(see Yaf_Controller_Abstract). that is a action method can also be a Yaf_Action_Abstract class.
Since there should be a entry point which can be called by Yaf (as of PHP 5.3, there is a new magic method __invoke, but Yaf is not only works with PHP 5.3+, Yaf choose another magic method execute), you must implement the abstract method Yaf_Action_Abstract::execute() in your custom action class.
$module
[, string $controller
[, string $action
[, array $paramters
]]] )