That's a very vague question, but OOP is used in the same way in PHP as it would be in any other scenario: as a way of modelling the interactions between discrete entities in your business logic.
A classic application for OOP in a web context would be a blog, where each post on the blog would be represented by an object, which in turn might have references to other objects representing the comments on that post.
As Jon said in his comment, though, OOP should be used only where it makes sense and logically fits into the system you're designing. Don't try to shoe-horn something into it just for the sake of it. Many PHP applications are simple enough not to require an OO approach, and are best implemented with a clean, straightforward procedural approach.
One caveat of OOP is that it can be quite difficult for beginners to learn, and is very easy to misunderstand. I started programming with PHP, and it took me a fair bit of trial and error to understand exactly how OOP should properly be used.