Although it is not normally necessary, you can
unset the singleton instance of the registry, if desired.
Use the static method _unsetInstance() to do so.
Data Loss Risk
When you use _unsetInstance(),
all data in the static registry are
discarded and cannot be recovered.
You might use this method, for example, if you want to
use setInstance() or setClassName()
after the singleton registry object has been initialized.
Unsetting the singleton instance allows you to use these methods
even after the singleton registry object has been set. Using
Zend_Registry in this manner is not recommended for typical
applications and environments.
Example 676. Example of _unsetInstance() Method Usage
<?php
Zend_Registry::set('index', $value);
Zend_Registry::_unsetInstance();
// change the class
Zend_Registry::setClassName('My_Registry');
Zend_Registry::set('index', $value);