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

The behavior is correct, when inheriting a static variable it means that all instances of the parent class and the descendants share the actual SAME variable so if an instance changes the the variable value, it appears to be changed or "overwritten" in all instances because it IS the same variable being shared. — Yaniro, Apr 03 '12 at 08:38

score 4 · Accepted Answer · answered Apr 03 '12 at 08:33

4

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.

answered Apr 03 '12 at 08:33

KingCrunch

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That makes sense. Is there not be a way to force the children to have their own version of the static variable without having to define it in each case? – Laurence Roberts Apr 03 '12 at 08:54
"Without defining it"? That _is_ the way to "force" it :? – KingCrunch Apr 03 '12 at 08:57
True, it just appears like they are not inheriting correctly, but I can see why it is like this. Thanks. – Laurence Roberts Apr 03 '12 at 09:02

PHP sibling classes static binding

1 Answers1