Arrays of objects

Hi, PHP provides a number of array-related functions which rely on a relation existing between each array element. This is easy to understand for primitive types (integers, strings and so on) but how does it work for objects? For example, I have a class Person. I have a number of Person objects in an array. How do I define how a Person object is 'equal' to another, for the purposes of searching through the array? Or would it be better to write a Collection class, encapsulating an array (or some other storage structure), which used callbacks in the Person class to determine equality, much in the way the Java equals() method works? Thanks, Dan