Best OOP tutorials I’ve encountered, so far:
http://net.tutsplus.com/tutorials/php/object-oriented-php-for-beginners/
— uses a house and a blue print as analogy.
http://www.killerphp.com/tutorials/object-oriented-php/index.php
— uses a person as an analogy.
http://www.tuxradar.com/practicalphp/6/0/0
— more in-depth approach and discussion.