PHP and Static Variables in Object Member Functions

Up until today, I thought I had a fairly good grasp of how the static modifier worked. I know that (in laymans terms) a static variable in a function does not 'reset' across calls to that function, and I know that static variables and functions on a class are accessible by calling upon them through the class itself (not an instantiation of the class).

My problem is this: today I found that if I declare a static variable inside of a non-static function on a class, all instantiations of that class share that static variable in separate calls to the member function.

For example:

class A {
    public function GetValue() {
        static $value = 0;
        $value++;
        return $value;
    }
}

$instance_1 = new A();
$instance_2 = new A();
echo $instance_1->GetValue();
echo $instance_1->GetValue();

echo $instance_2->GetValue();
echo $instance_2->GetValue();

echo $instance_1->GetValue();
echo $instance_1->GetValue();

Notice that the GetValue function is neither declared as static or used in a static way (as in, called on the class itself).

Now, I always assumed that this would output: 121234

Instead, I find that it will output: 123456

Like I say, I would understand this if the static variable $value was inside of a static function. However, with it being inside a non-static function I just assumed that it would only be 'tied' to the function 'within' each individual instantiation.

I guess my question is twofold, then... 1) is this a bug or expected behaviour? 2) do other languages treat these 'static inside non-static' variables the same way, or is this unique to PHP?