Static class variable for all instances

I just got a question on Aardvark asking if you could somehow share a variable value between multiple instances of the same class. Since the value is an array you could not do this with a class constant.

This could instead be solved by declaring a static variable in the class. That would make it available in all instances of the class. You could then also control encapsulation by declaring it public, protected or private.

Because I took the time to assemble a proof of concept, I felt like sharing it here for future reference.

class MyClass
    public static $x = 'x';

$a = new MyClass;
$b = new MyClass;

echo sprintf("%s %s\n", $a::$x, $b::$x);
// prints "x x"

$a::$x = 'y';

echo sprintf("%s / %s\n", $a::$x, $b::$x);
// prints "y y"