在方法的參數(shù)中有個(gè)event是什么意思

// PHP 代碼
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();
        }
    }
}

代碼如上,請(qǐng)問(wèn)上述回調(diào)方法中$event參數(shù)之前的 event 是什么意思? 有什么作用?