PHP: Patterns - Manual

Patterns

Patterns are ways to describe best practices and good designs. They show a flexible solution to common programming problems.

Factory

The Factory pattern allows for the instantiation of objects at runtime. It is called a Factory Pattern since it is responsible for "manufacturing" an object. A Parameterized Factory receives the name of the class to instantiate as argument.

Example #1 Parameterized Factory Method


<?php

class Example

{

    // The parameterized factory method

    public static function factory($type)

    {

        if (include_once 'Drivers/' . $type . '.php') {

            $classname = 'Driver_' . $type;

            return new $classname;

        } else {

            throw new Exception ('Driver not found');

        }

    }

}

?>

Defining this method in a class allows drivers to be loaded on the fly. If the Example class was a database abstraction class, loading a MySQL and SQLite driver could be done as follows:


<?php

// Load a MySQL Driver

$mysql = Example::factory('MySQL');



// Load a SQLite Driver

$sqlite = Example::factory('SQLite');

?>

Singleton

The Singleton pattern applies to situations in which there needs to be a single instance of a class. The most common example of this is a database connection. Implementing this pattern allows a programmer to make this single instance easily accessible by many other objects.

Example #2 Singleton Function


<?php

class Example

{

    // Hold an instance of the class

    private static $instance;

    

    // A private constructor; prevents direct creation of object

    private function __construct() 

    {

        echo 'I am constructed';

    }



    // The singleton method

    public static function singleton() 

    {

        if (!isset(self::$instance)) {

            $c = __CLASS__;

            self::$instance = new $c;

        }



        return self::$instance;

    }

    

    // Example method

    public function bark()

    {

        echo 'Woof!';

    }



    // Prevent users to clone the instance

    public function __clone()

    {

        trigger_error('Clone is not allowed.', E_USER_ERROR);

    }



}



?>

This allows a single instance of the Example class to be retrieved.


<?php

// This would fail because the constructor is private

$test = new Example;



// This will always retrieve a single instance of the class

$test = Example::singleton();

$test->bark();



// This will issue an E_USER_ERROR.

$test_clone = clone $test;



?>