Here, I'm not going to offer a full reference for what each design pattern is. You can buy a book to learn that, or even do some "googling" and find other helpful resources that probably will treat the subject much more extensively. Instead, I'll provide you with a concise explanation of what an Iterator is, in conjunction with numerous sample codes, which hopefully will help you understand much more easily how it can applied in PHP object-oriented programming.
They start off by looking at what the Iterator pattern is - the setup of a pseudo-class and an explaination of how it works. They follow this with a functional example and creating a subclass off of the generic base class.