Description:
------------
It's possible to define a reference to a non-existent variable, which 
gets assigned in a loop (example 1). It's possble to define a reference 
to an existent key, too. Why is it not possible to define references to 
non existent respective reference defined keys?

Reproduce code:
---------------
// Example 1:
$a = array (1, 2, 3);
$b = &$value;
	
foreach ($a as $value) {
	echo $b;
}

// Example 2:
$a = array(array(1), array(2), array(3));

foreach ($a as $value) {
	$b = &$value[0];
	echo $b;
}

// Example 3:
$b = &$value[0];

foreach ($a as $value) {
	echo $b;
}

Expected result:
----------------
123
123
123

Actual result:
--------------
123
123
(nullnullnull)