echo "I {$this->name} know {$this->skill}" . PHP_EOL;
    }
    public function override()
    {
        parent::override();
        echo "overridden!" . PHP_EOL;
    }
}
$foo = new Person("foo", 20);
$bar = new Developer("bar", 30, "PHP");
$foo->introduce();
$bar->introduce();
$bar->knows();
$foo->override();
$foo->cannotOverride();
$bar->override();
$bar->cannotOverride();
// abstract class
abstract class ANinja
{
    public abstract function kill(Person $someone);
    protected abstract function isOk();
    public function knowledge()
    {
        echo "this is a common method, already implemented!" . PHP_EOL;
    }
}
class Ninja extends ANinja
{
    public function kill(Person $someone)
    {