As far as books, most OOP PHP programmers recommend: "Objects, Patterns, and Practice" by author Matt Zandstra, which is a great book. But I think "Object Oriented Solutions" by author David Powers is by far the best of the best. It's range is absolute beginners all the way to professionals. It's all you would ever need on any level, it's written brilliantly by a great teacher (not just a great programmer), and everything is crystal clear - you're never lost.

As far as online manuals, I've never seen any that you would learn much from. Good for reference, but not for actually conceptually learning and mastering the skill.

There are some great PHP videos on Lynda.com and Killer.php, including OOP PHP. They're not free, but they're well worth the small amount they charge. It's a steal for the unbelievable amount of high quality material and fantastic teachers on those sites.

All in all, if you don't mind reading, the books I recommended are the way to go if you want to thoroughly learn it inside out.