I'm developing a web system using regular PHP. This was my first experience with PHP so the code is not legible nor clean. It mixes some HTML code with PHP.
I'd say I have already done half of the code.
What are the real advantages of the Object-oriented PHP?
The website is about books and book authors, using MySQL and Apache. So it's not a very complicated website.
The real advantage of object-orientation: your code is better organized, easier to maintain, more modular (and thus easier to reuse), and potentially less brittle (because of encapsulation of state and implementation, and hopefully better security). (The cynic in me also says that if you learn object-oriented PHP, you take the first important step to leaving the PHP ghetto. Heh. Worked for me!)
There's already a lot of questions from PHPers moving into OO on Stack Overflow:
Not to mention that there are zillions of PHP object-oriented tutorials out there. My take: basically, yes, if you are writing PHP, you should probably be writing object-oriented PHP for anything beyond the most trivial of applications. There are lots of Rails-like frameworks for PHP that will make your life easier, and may help you become a better programmer.