PHP-General - PHP Multiple application best practices

Here is something I never had to deal with yet(and don't really plan on it anytime soon), however I would like the opinions of the PHP developers on here of how they handle this type of scenario?

Scenario:
You are a web developer, and a client has contracted you to deploy a full scale customer facing website for their company. They gave you specific applications they want to deploy and integrated together seamlessly all under a single template their crafty HTML5/CSS3 designers provided. Templates are run of the mill with no AJAX/jQuery, just a normal website with a header, footer, content DIV and navigation area. Here are the web apps they want to deployed:

1. Joomla(for content management)
2. vBulletin or phpBB
3. osCommerce
4. osTicket

Here's the catch: the client would like the forum, Shopping Cart, and ticketing system to all use the same authentication for customers, one login for all systems, as most customers would not want to juggle 3 IDs for one website, that's ridiculous.

In my past experiences with PHP and what some developers have noted, is that integrating different PHP applications together to all sign in using a single authentication system, is very difficult and time consuming. This is what I see as being one of the larger pitfalls of PHP coding. The more complex the website is, the more difficult it is to integrate each app and make sure one upgrade doesn't break another.

The ideal solution I see, is to use one PHP framework which includes it's own Authentication backend and API for it to be used by multiple apps running on the framework. The unfortunate bit here, is that most PHP applications do not have a pluggable authentication system. Kudos to those that do. Another problem, is that most of the current popular PHP projects out there are bound directly to PHP itself, and not a PHP framework such as Cake, for example.

What are your opinions and possible solution for this scenario?