PHP sibling classes static binding

Question

When autoloading in multiple classes that extend the same parent, they seem to overwrite each others static variables.

Using the code below, if the $staticvar is only defined in the parent Controller class then Foo::$staticvar is overwritten by the subsequent called classes that also extend Controller.

If however Foo itself also defines $staticvar = null; then it is not overwritten. Why is this?

---

System.php

class System {
    static function load() {
        spl_autoload_register('System::autoload_controller');
        $classes = array('Foo', 'Bar', 'Test');
        foreach ($classes as $name) {
            $instance = new $name;
        }
    }

    static function autoload_controller($name) {
        echo $name.":\n";
        require_once strtolower($name).'.php';
        $name::$staticvar = 'static_'.$name;

        echo "Foo is: ".Foo::$staticvar."\n";
        echo $name." is: ".$name::$staticvar."\n\n";
    }
}

class Controller {
    static $staticvar = null;
}

System::load();

---

If foo.php is this:

class Foo extends Controller {

}

I get the output:

Foo:
Foo is: static_Foo
Foo is: static_Foo

Bar:
Foo is: static_Bar
Bar is: static_Bar

Test:
Foo is: static_Test
Test is: static_Test

---

But if I change foo.php to this:

class Foo extends Controller {
    static $staticvar = null;
}

I get the output:

Foo:
Foo is: static_Foo
Foo is: static_Foo

Bar:
Foo is: static_Foo
Bar is: static_Bar

Test:
Foo is: static_Foo
Test is: static_Test

Answer 1

If however Foo itself also defines $staticvar = null; then it is not overwritten. Why is this?

Because "static" means, it is static (bound) to the scope (class) where it is defined. This means Controller::$staticvar and Foo::$staticvar are two different properties.