php-doc-en/language/oop5/patterns.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.9 $ -->
<sect1 id="language.oop5.patterns">
 <title>Patterns</title>
 <para>
  Patterns are ways to describe best practices and good designs.
  They show a flexible solution to common programming problems.
 </para>

 <sect2 id="language.oop5.patterns.factory">
  <title>Factory</title>
  <para>
   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.
  </para>
  <example>
   <title>Parameterized Factory Method</title>
   <programlisting role="php">
<![CDATA[
<?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');
        }
    }
}
?>
]]>
   </programlisting>
   <para>
    Defining this method in a class allows drivers to be loaded on the
    fly. If the <literal>Example</literal> class was a database
    abstraction class, loading a <literal>MySQL</literal> and
    <literal>SQLite</literal> driver could be done as follows:
   </para>
   <programlisting role="php">
<![CDATA[
<?php
// Load a MySQL Driver
$mysql = Example::factory('MySQL');

// Load a SQLite Driver
$sqlite = Example::factory('SQLite');
?>
]]>
   </programlisting>
  </example>
 </sect2>

 <sect2 id="language.oop5.patterns.singleton">
  <title>Singleton</title>
  <para>
   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.
  </para>
  <example>
   <title>Singleton Function</title>
   <programlisting role="php">
<![CDATA[
<?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);
    }

}

?>
]]>
   </programlisting>
   <para>
    This allows a single instance of the <literal>Example</literal>
    class to be retrieved.
   </para>
   <programlisting role="php">
<![CDATA[
<?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;

?>
]]>
   </programlisting>
  </example>
 </sect2>

</sect1>
 
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