在方法的参数中有个event是什么意思

Question

// PHP 代码
interface process {
    public function process() {
        echo 'aaaaa';
    }
}

class event {
    private $m;
    public function __construct($me) {
        $this->m = $me;
    }
    public function click () {
        echo 'ffff';
    }
}

class playProcess {
    private $msg = null;
    public function __construct() {
    }
    public function callback (event $event) {
        $this->msg = $event->click();
        if ($this->msg instanceof process) {
            $this->msg->process();
        }
    }
}

代码如上,请问上述回调方法中$event参数之前的 event 是什么意思? 有什么作用?