In a new post to his "Quaint Tech" blog, Paul van Iterson talks about why you want (to use) object-oriented programming in your PHP applications versus the more procedural methods.
Most modern programming languages are based on an OO approach and don't even allow for procedural coding anymore (for instance in languages as Java, C++, C#, Python, and Ruby). Explaining OOP goes beyond the scope of this article, so for understanding OOP in PHP I'd like to point you in this direction. For now let's just say that OOP is about creating relatively small objects with specific functions that interact with each other to form a complete application.
Right off, he lists two advantages to using OOP methods (reusability and better structure) and a disadvantage (can be more time consuming to write). He does still recommend it though to any "serious PHP developer taking on serious projects". He illustrates the point with some of his own experience - an application for a real estate firm.