simple -- abstract

<?php
abstract class Operation {
    protected $_NumberA = 0;
    protected $_NumberB = 0;
    protected $_Result = 0;

    public function __construct($A, $B){
        $this->_NumberA = $A;
        $this->_NumberB = $B;
    }

    public function clearResult(){
        $this->_Result = 0;
    }

    abstract protected function getResult();

}
class OperationAdd extends Operation {

    public function getResult(){
        $this->_Result = $this->_NumberA + $this->_NumberB;
        return $this->_Result;
    }

}
class OperationSub extends Operation {

    public function getResult(){
        $this->_Result = $this->_NumberA - $this->_NumberB;
        return $this->_Result;
    }
}
class OperationFactory {

    //创建保存实例的静态成员变量
    private static $obj;

    //创建访问实例的公共的静态方法
    public static function CreateOperation($type, $A, $B){
        switch($type) {
            case '+':
                self::$obj = new OperationAdd($A,$B);
                break;
            case '-':
                self::$obj = new OperationSub($A,$B);
                break;
        }
        return self::$obj;
    }
}
$obj = OperationFactory::CreateOperation('-', 5, 6);
echo $obj->getResult();