问面向对象和对象公有属性的继承问题？

class NBAplayer {

    public static $president = "david stern";

}

$tao = new NBAplayer('tao');

$jun = new NBAplayer('jun');

echo NBAplayer::$president; // david stern

class Human {

    public $name;

    public $president = 'david stern';

}

class NBAplayer extends Human {

    function __construct($name){

        $this ->name = $name;

    }

}

$tao = new NBAplayer('tao');

$jun = new NBAplayer('jun');

echo $tao ->president;

echo $jun ->president;