父类引用仅存在于子元素中的属性是否合适？

In the project my team is currently working on, we're modifying a commercial PHP application. The app is strewn with code where a parent class checks for and works with a property that doesn't exist in the parent class, like so:

class A 
{
    function doSomething()
    {
        if (property_exists($this, 'some_property'))
        {
            $this->some_property = $_REQUEST['val'];
        }
    }

}

class B extends A
{
    protected $some_property;

    function doSomething()
    {
        parent::doSomething();
    }
}

We feel vaguely dirty having to modify this code; is this proper design? What are the ways (other than the obvious) something like this can be avoided?

Relying upon methods that must exist in a subclass is not dirty, as long as you can declare them as abstract.

However, it is not good practice to rely on and manipulate properties outside of a class. It's best to use abstract setters, like this:

abstract class A 
{
    abstract protected function setSomeProperty($data);

    public function doSomething()
    {
        $this->setSomeProperty($_REQUEST['val']);
    }
}

class B extends A
{
    private $some_property;

    public function doSomething()
    {
        parent::doSomething();
    }

    protected function setSomeProperty($data)
    {
        $this->some_property = $data;
    }
}

More info here: PHP Class Abstraction

However, since you said you're not allowed to modify the parent class, I would suggest making a subclass that acts as an Adapter to what the parent class "expects", and a class that you're able to design "properly".

You can create a virtual method hook in the parent class which can later be overridden by children.

I think it's more neat to create a sub-class, where all members have function doSomething(). In that case you don't create a not-working function in a parent class (with eventual hacks), but still have the general "super-function".

class A 
{


}
class C extends A {

    protected $some_property;

    function doSomething()
    {
        $this->some_property = $_REQUEST['val'];
    }
}

class B extends C
{
    protected $some_property;

    function doSomething()
    {
        parent::doSomething();
    }
}

You might consider abstracting the parent class. So the methods that the children must have are declared in the parent, but not implemented.