For those who have yet to make the leap over from PHP4 to PHP5, it'd be worth your time to check out this new article from PHPBuilder.com detailing some of the object-oriented features that are new to this more advanced version.
With the release of PHP 5 in 2004 there was plenty of room for improv- ing PHP's OO capabilities. At this point, Java, the most popular OO language to date, had already been around for almost 10 years. Why did it take PHP so long to become a full-fledged OO language? The short answer is because PHP is principally a web development language and the pressures of web development have only recently pushed it in this direction.
Their "long answer" goes through each of the changes that have been made (like access modifiers, built-in classes, web services, and reflection classes) explaining them in the context of comparing them back to PHP4. They also include a "where to go from here" section to advise on how to take that first step to PHP5-ness.