unset does not effect the object which is outside the method context

gistfile1.php

<?php
    /**
     * Delete an entity from the global identity map and all other queues
     * @param AbstractDomainEntity $entity
     */
    static public function unRegister(AbstractDomainEntity $entity)
    {
        $instance = self::getInstance();
        $instance->registerClean($entity);
        unset($instance->all[$instance->globalKey($entity)]);
        unset($entity);
    }