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# 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 19-24. 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 19-25. 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); ?>`

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