Object Aggregation
PHP Manual

Introduction

Warning

This extension is EXPERIMENTAL. The behaviour of this extension including the names of its functions and any other documentation surrounding this extension may change without notice in a future release of PHP. This extension should be used at your own risk.

In Object Oriented Programming, it is common to see the composition of simple classes (and/or instances) into a more complex one. This is a flexible strategy for building complicated objects and object hierarchies and can function as a dynamic alternative to multiple inheritance. There are two ways to perform class (and/or object) composition depending on the relationship between the composed elements: Association and Aggregation.