Peculiar Behaviour with PHP (5.3), static inheritance and references

I'm writing a library in PHP 5.3, the bulk of which is a class with several static properties that is extended from by subclasses to allow zero-conf for child classes.

Anyway, here's a sample to illustrate the peculiarity I have found:

<?php

class A {
    protected static $a;
    public static function out() { var_dump(static::$a); }
    public static function setup($v) { static::$a =& $v; }
}
class B extends A {}
class C extends A {}

A::setup('A');
A::out(); // 'A'
B::out(); // null
C::out(); // null

B::setup('B');
A::out(); // 'A'
B::out(); // 'B'
C::out(); // null

C::setup('C');
A::out(); // 'A'
B::out(); // 'B'
C::out(); // 'C'

?>

Now, this is pretty much desired behaviour for static inheritance as far as I'm concerned, however, changing static::$a =& $v; to static::$a = $v; (no reference) you get the behaviour I expected, that is:

'A'
'A'
'A'

'B'
'B'
'B'

'C'
'C'
'C'

Can anyone explain why this is? I can't understand how references effect static inheritance in any way :/

Update:

Based on Artefacto's answer, having the following method in the base class (in this instance, A) and calling it after the class declarations produces the behaviour labelled as 'desired' above without the need to assign by reference in setters, whilst leaving the results when using self:: as the 'expected' behaviour above.

/*...*/
public static function break_static_references() {
    $self = new ReflectionClass(get_called_class());
    foreach($self->getStaticProperties() as $var => $val)
        static::$$var =& $val;
}
/*...*/
A::break_static_references();
B::break_static_references();
C::break_static_references();
/*...*/