<p>I'm a PHPer, and am not writing object-oriented code.</p>

<p>What are the advantages of OO over procedural code, and where can I learn how to apply these ideas to PHP?</p>

Objects help keep your code isolated between different sections, so that if you need to make a change to one section you can be confident it won't affect other sections: loose coupling.
Then, when you've done that for a while, you'll start finding that objects you created for one app are also useful in others, and you start getting better code re-use as well.  So the new app has part of the work already done, and is using time-tested code:  software is built faster with fewer bugs.