The Singleton Design Pattern for PHP

By Tony Marston
2005-07-29


Introduction

In software engineering, a design pattern is a general solution to a common problem in software design. A design pattern isn't a finished design that can be transformed directly into code, it is a description or template for how to solve a problem that can be used in many different situations.

One of these design patterns is called the Singleton. It's purpose can be described quite briefly as follows:

Ensure a class has only one instance and provide a global point of access to it.

It achieves this by only creating a new instance the first time it is referenced, and thereafter it simply returns the handle to the existing instance.

Why should you want this facility in the processing of a web page? There are several reasons:

• You have references to an object in multiple places, and you do not want the overhead of creating a new instance of that object for each reference.
• You may wish to pass the object's state from one reference to another rather than always starting from an initial state.

As with all design patterns there is more than one way in which it can be implemented, so in the following sections I will document some of the methods that I have encountered.

You should also be aware that the code samples in this document will work in both PHP 4 and PHP 5.

Tutorial Pages:
» Introduction
» A non-class Helper function
» A separate Helper method within each class
» A single Helper method for all classes
» References
» Conclusion

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