php--static关键字

class Foo
{
    public static $my_static = 'foo';
    public function staticValue(){
        return self::$my_static;
    }
}
class Bar extends Foo
{
    public function fooStatic(){
        return parent::$my_static;
    }
}
print Foo::$my_static.&#39;<br>&#39;;
$foo = new Foo();
print $foo->staticValue().&#39;<br>&#39;;
print $foo->my_static.&#39;<br>&#39;;
print $foo::$my_static.&#39;<br>&#39;;
$classname = &#39;Foo&#39;;
print $classname::$my_static;
print Bar::$my_static.&#39;<br>&#39;;
$bar = new Bar();
print $bar->fooStatic().&#39;<br>&#39;;