<?php
class test2 {
    private static $instances = 0;
    public $instance;

    public function __construct() {
        $this->instance = ++self::$instances;
    }

}

$foo = new test2();

if (is_object($foo)) {
    class test2_child extends test2 {

    }
}

$child = new test2_child();

echo $foo->instance . "\n";
echo $child->instance . "\n";
?>