Take a look at the following code:

function b() 
{
	global $a;
	$b = 'bar';
	$a = &$b;
}

$a = 'foo';
b();
echo $a;

What do you think is the value of $a after calling b()? The correct answer is 'foo'. If we remove the ampersand in '$a = &$b;' the value will be: 'bar'.

This behaviour can be explained as following:

In function b() the 'global $a' statement creates new a local variable $a as a reference to the global $a variable. '$a = &$b' then changes the local $a variable as a reference to $b. The global $a variable is not touched.

To achieve a sane result use the following construction, create the reference first, then assign a new value:

function b() 
{
	global $a;
	$b = &$a
	$b = 'bar';
}

I still think this should be considered flawed implementation, if not simply broken.