Note: I am assuming by 'PHP to PHP5' you mean from PHP4 to PHP5. PHP5 is not a different languages from PHP, it is simply a different version.
PHP4 to PHP5
Here is the main differences between PHP4 and PHP5.
http://www.php.net/manual/en/migration5.php
The specific OOP changes are featured in the Object Model
http://www.php.net/manual/en/migration5.oop.php
One of the places that page points that will give you the basics to how OOP works in PHP would be the Classes and Object reference page:
http://www.php.net/manual/en/language.oop5.php
PHP Class Examples
If you want some specific examples of how OOP is used in PHP, you might want to look at my answer on some basic classes you might use in a web development setting:
https://stackoverflow.com/questions/2035449/why-is-oop-hard-for-me/2035482#2035482
Learning PHP
To learn PHP, you can do a search on PHP and OOP on StackOverflow, and the first few results give you resources to do so:
https://stackoverflow.com/search?q=php+oop
If you want to skip clicking the link, you can just go to http://php.net, cause I guarentee that almost all of them point there. PHP has one of the best documentations out there, and it is extremely easy to use, and you will return to it very, very often.
For the record, the first result talks about how to learn object-oriented programming in php.
