PHP工厂模式demo

<?php
//工厂模式

interface operstion{
    function oper($a,$b);
}
//加
class add implements operstion{
    function oper($a,$b){
       echo $c = $a + $b;
    }
}
//减
class subtract implements operstion{
    function oper($a,$b){
        echo $d = $b - $a;
    }
}
//乘
class multiply implements operstion{
    function oper($a,$b){
        echo $e = $a * $b;
    }
}
//除
class divide implements operstion{
    function oper($a,$b){
        $f = $b / $a;
    }
}
class Factory{
    static public function createDb($type){
        switch($type){
            case 'add':
                return new add();
                break;
            case 'subtract':
                return new subtract();
                break;
            case 'multiply':
                return new multiply();
                break;
            case 'divide':
                return new divide();
                break;
        }
    }
}

$obj = Factory::createDb('add');
$obj->oper(2,4);