Most functions require the current connection as its first parameter. This leads to a lot of typing, which is quite often not necessary if there is just one open connection. A default connection will improve this.

You've learned a lot about PHP. You now know how to create dynamic loadable modules and statically linked extensions. You've learned how PHP and Zend deal with internal storage of variables and how you can create and access these variables. You know quite a set of tool functions that do a lot of routine tasks such as printing informational texts, automatically introducing variables to the symbol table, and so on.

It is strongly recommended that you include the class definitions of all such registered objects on all of your pages, even if you do not actually use these classes on all of your pages. If you don't and an object is being unserialized without its class definition being present, it will lose its class association and become an object of class stdClass without any functions available at all, that is, it will become quite useless.

Before a new sibling is added it is first duplicated. Therefore the new child is a completely new copy which can be modified without changing the node which was passed to this function. If the node passed has children itself, they will be duplicated as well, which makes it quite easy to duplicate large parts of a xml document. The return value is the added sibling. If you plan to do further modifications on the added sibling you must use the returned node.