PHP function to cast an object from one class to another.

function.cast.php

<?php
/**
 * Cast an object into a different class.
 *
 * Currently this only supports casting DOWN the inheritance chain,
 * that is, an object may only be cast into a class if that class 
 * is a descendant of the object's current class.
 *
 * This is mostly to avoid potentially losing data by casting across
 * incompatable classes.
 *
 * @param object $object The object to cast.
 * @param string $class The class to cast the object into.
 * @return object
 */
function cast($object, $class) {
	if( !is_object($object) ) 
		throw new InvalidArgumentException('$object must be an object.');
	if( !is_string($class) )
		throw new InvalidArgumentException('$class must be a string.');
	if( !class_exists($class) )
		throw new InvalidArgumentException(sprintf('Unknown class: %s.', $class));
	if( !is_subclass_of($class, get_class($object)) ) 
		throw new InvalidArgumentException(sprintf(
			'%s is not a descendant of $object class: %s.',
			$class, get_class($object)
		));

	/**
	 * This is a beautifully ugly hack.
	 *
	 * First, we serialize our object, which turns it into a string, allowing
	 * us to muck about with it using standard string manipulation methods.
	 *
	 * Then, we use preg_replace to change it's defined type to the class
	 * we're casting it to, and then serialize the string back into an
	 * object.
	 */
	return unserialize(
		preg_replace(
			'/^O:\d+:"[^"]++"/', 
			'O:'.strlen($class).':"'.$class.'"',
			serialize($object)
		)
	);
}
?>

To initialize properties on the new object that didn't exist in the old one (if the new extends the old), its class definition needs to have a __wakeup() method as unserialize doesn't call __construct().

Line 23 could be rewritten to allow either class or $object class to be a descendant of either:

        if (!is_subclass_of($class, get_class($object)) && !is_subclass_of(get_class($object), $class)) {
            throw new InvalidArgumentException(sprintf(
                '%s is not a descendant of $object class: %s.',
                $class,
                get_class($object)
            ));
        }