PHP How can I design this better using OO and SOLID principles?

[Blmz]

I've found lots of similar posts about related questions, this reply by taylonr came close but didn't quite answer my scenario. Need some help with the Solid Principles. https://adevait.com/software/solid-design-principles-the-guide-to-becoming-better-developers

Assume I have an interface:

public interface IShape

{

    decimal GetArea();

}

I then create 3 classes using this interface

public class Rectangle : IShape
{
    public decimal GetArea()
    {
        ...
    }
}

public class Triangle : IShape
{
    public decimal GetArea()
    {
        ...
    }
}

public class Circle : IShape
{
    public decimal GetArea()
    {
        ...
    }
}

I now want to add a int GetNumberOfSides() function (or some other function which is only relevant to some of my IShape's). Obviously this isn't relevant to the Circle class. If I then had a List<IShape> objects and wanted to iterate over call this function on relevant IShapes, how would you using object oriented design principles go about addressing this?

I could add a boolean to my IShape interface bool HasSides { get; set; } and run any logic of this, however I would need to cast to a specific class in order to access a GetNumberOfSides() function. I know this isn't right as it doesn't feel right, but I'm not sure how to do this.

I've thought about having another interface ISidedShape which inherited from IShape, but then back to iterating of my list how would I know which of the shapes had this specific method?

Any help would be much appreciated.

Thanks

 

[Ghost]

Honestly, the simplest thing to do in my opinion would be to just handle your return types.
A circle does have an integer of # of straight sides. That number is 0. A circle has 0 straight sides. This means that your number of sides function *can* actually be used for the circle, but the answer is 0. You could check the return for === 0 for 0 sides or perhaps return false if a shape (like a circle) is not "eligible" for counting straight sides. It's up to you... as long as your code knows the returns, I see no reason why you couldn't just return 0 for "how many sides?" on a circle.

Layman's terms:
-> function counts how many fries are left on your plate
-> (triangle) pass a plate with 3 fries to your function, it returns 3
-> (square) pass a plate with 4 fries, it returns 4
-> (circle) pass an empty plate to your function, it returns === 0 (0 fries/sides)
-> (empty) pass nothing to your function, it returns === false (ineligible)