php中的类替换

class car {
        function __construct($type){
                switch ($type) {
                        case 'big' : return new big_car();
                        case 'small' : return new small_car();
                }
        }
        function whatisit () {
                echo "this is car ;( n";
        }
}

class big_car extends car {
        function __construct(){}
        function whatisit () {
                echo "this is big car ;) n";
        }
}

class small_car extends car {
        function __construct(){}
        function whatisit () {
                echo "this is small car ;) n";
        }
}

所以目标是以这种方式使用它：

$mycar = new car('big');
$mycar->whatisit(); // i want it to say that it's big

我非常想说它的方法不好而且它不能以这种方式工作但是也许有一个技巧？

PS ::我知道我可以使用特殊的静态方法,但……