php中self关键字的功能是什么

可能重复：
PHP：自我vs.$this

它来自php手册，请让我知道我在哪里以及为什么使用self-keyword

<?php
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 . "'n";
$foo = new Foo();
print $foo->staticValue() . "'n";
print $foo->my_static . "'n";      // Undefined "Property" my_static 
print $foo::$my_static . "'n";
$classname = 'Foo';
print $classname::$my_static . "'n"; // As of PHP 5.3.0
print Bar::$my_static . "'n";
$bar = new Bar();
print $bar->fooStatic() . "'n";
?>