Singleton Sample Code

In Object Oriented Programming the Singleton design pattern answers to the problem of having a single or unique instance of a class in a program.

For example, as part of a dynamic web application, a database connection to the server is unique.
To preserve this uniqueness, it makes sense to use an object that takes the form of a singleton to create the sole representative object access to the database, and store the reference to it in a global variable of the program so that we can access it from anywhere in the script.

The Singleton design pattern implementation has three characteristics:     

1. A private static attribute that retain the unique instance of the class.     
2. A private constructor to prevent the creation of items from outside the class     
3. A static method to either instantiate the class is to return the unique instance created.

The code below shows a minimal class that incorporates the Singleton design pattern. We find the minimum, the three characteristics just presented above.

class Singleton 
{
 
  /**
   * @var Singleton
   * @access private
   * @static
   */
   private static $instance = null;
 
   /**
    * Private constructor: only to be created by myself
    *
    * @param void
    * @return void
    */
   private function __construct() 
   {  
   }
 
   /**
    * If it doesn't exits then creates one unique instance 
    * of this class, otherwise it returns the existant one.
    *
    * @param void
    * @return Singleton
    */
   public static function getInstance() 
   {
     // If I am not created yet then I create myself 
     if(is_null(self::$instance)) {
       self::$instance = new Singleton();  
     }
 
     return self::$instance;
   }
}

// Try to get an instance of this class
Singleton::getInstance();

 I've seen many uses of this design pattern, just a little word of warning: do not fall into "Singletonitis": willing to use  this pattern for all your constants in your code. :)