Why do they do it?

Question

PHP

Why create an interface if the methods will work anyway?
Why do this:

interface Walking{
    public function walk();
}
interface Swimming{

    public function swim();
}
class Dog extends Animal implements Walking {

    public function walk()
    {
        // TODO: Implement walk() method.
    }
}
class Penguin extends Animal implements Walking, Swimming{

    public function walk()
    {
        // TODO: Implement walk() method.
    }

    public function swim()
    {
        // TODO: Implement swim() method.
    }
}

If possible like this:

class Dog extends Animal{

    public function walk()
    {
        // TODO: Implement walk() method.
    }
}
class Penguin extends Animal{

    public function walk()
    {
        // TODO: Implement walk() method.
    }

    public function swim()
    {
        // TODO: Implement swim() method.
    }
}

Answer 1

An interface GUARANTEES the existence of such a method in a class.