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PHP Don't allow object to instantiate more than once
function __construct() {
         self::$_instance =& $this;

         if (!empty(self::$_instance)) {
            foreach (self::$_instance as $key => $class) {
                     $this->$key = $class;
            }
         }
}

class MyClass {
    private static $instance = null;
    private final function __construct() {
        //
    }
    private final function __clone() { }
    public final function __sleep() {
        throw new Exception('Serializing of Singletons is not allowed');
    }
    public static function getInstance() {
        if (self::$instance === null) self::$instance = new self();
        return self::$instance;
    }
}

class Foo {
    private static $instance;

    private function __construct() {
    }

    public static function getInstance() {
        if (!isset(static::$instance)) {
            static::$instance = new static();
        }

        return static::$instance;
    }
}