How do you organize and manage your helper objects like the database engine, user notification, error handling and so on in a PHP based, object oriented project?

Say I have a large PHP CMS. The CMS is organized in various classes. A few examples:

• the database object
• user management
• an API to create/modify/delete items
• a messaging object to display messages to the end user
• a context handler that takes you to the right page
• a navigation bar class that shows buttons
• a logging object
• possibly, custom error handling

etc.

I am dealing with the eternal question, how to best make these objects accessible to each part of the system that needs it.

my first apporach, many years ago was to have a $application global that contained initialized instances of these classes.

global $application;
$application->messageHandler->addMessage("Item successfully inserted");

I then changed over to the Singleton pattern and a factory function:

$mh =&factory("messageHandler");
$mh->addMessage("Item successfully inserted");

but I'm not happy with that either. Unit tests and encapsulation become more and more important to me, and in my understanding the logic behind globals/singletons destroys the basic idea of OOP.

Then there is of course the possibility of giving each object a number of pointers to the helper objects it needs, probably the very cleanest, resource-saving and testing-friendly way but I have doubts about the maintainability of this in the long run.

Most PHP frameworks I have looked into use either the singleton pattern, or functions that access the initialized objects. Both fine approaches, but as I said I'm happy with neither.

I would like to broaden my horizon on what is possible here and what others have done. I am looking for examples, additional ideas and pointers towards resources that discuss this from a long-term, real-world perspective.

Also, I'm interested to hear about specialized, niche or plain weird approaches to the issue.

Bounty

I am following the popular vote in awarding the bounty, the answer which is probably also going to give me the most. Thank you for all your answers!