Domain modeling base class

Implemented lazy loading and registration as the only one
Java needs to introduce the concepts of factories and warehouses, while PHP only needs two or three lines of code to complete the same function

/**
 * Domain model parent class
 * Implement delayed data loading and registration as unique
 * @author liuxu
 *
*/
abstract class Domain
{

 protected $_id;
 protected $_id2;

 protected $_fields = array();

 protected $_loadedById = false;

 static protected $_objs;

 private function __construct($id,$id2=0)
 {
 $this->_id = $id?$id:0;
 $this->_id2 = $id2?$id2: 0;
 }

 static public function factory($id,$id2=0)
 {
 $class = get_called_class();
 if(!isset(self::$_objs[$class][$id][$ id2]))//Register as unique
 {
 self::$_objs[$class][$id][$id2] = new $class($id,$id2);
 }

 return self::$ _objs[$class][$id][$id2];
 }

 public function load($fields=array())
 {
 if($fields)
 {
 foreach($fields as $key=> $value)
 {
 $this->_fields[$key] = $value;
 }
 }
 }

 public function wash()
 {
 $this->_fields = array();
 }

 //Load complete data
 abstract protected function loadById();

 public function __get($field)
 {
 if(!isset($this->_fields[$field]) && !$this-> _loadedById)
 {
 //Lazy loading
 $this->loadById();
 $this->_loadedById = true;
 }

 $value = isset($this->_fields[$field]) ?$this->_fields[$field]:null;
 return $value;
 }



}