When two singletone classes calls getInstace methods of each other

Hi. Have you ever had such problem when two different classes gets each other instances in their constructors. In this case PHP always does not see a static variable which should keep an object. And some kind of infinite recursion starts which causes that php returns nothing (i think because it kills a thread).

The code:

class first
{
    static function &getInstance()
    {
        static $instance;
        if (!isset($instance)) 
        {
            $instance = new first();
        }
        return $instance;
    }
    
    function __construct()
    {
        echo 'initiated '.__CLASS__.'<br />';
        $s = &second::getInstance();      
    }
}

class second
{
    static function &getInstance()
    {
        static $instance;
        if (!isset($instance)) 
        {
            $instance = new second();
        }
        return $instance;
    }
    
    function __construct()
    {
        $f = &first::getInstance();
        echo 'initiated '.__CLASS__.'<br />';    
    }
}

$f = &first::getInstance();
$s = &second::getInstance();

$f = &first::getInstance();
$s = &second::getInstance();

PHP:

The same example with $GLOBALS

class first
{
    static function &getInstance()
    {
        if (!isset($GLOBALS['first'])) 
        {
            $GLOBALS['first'] = new first();
        }
        return $GLOBALS['first'];
    }
    
    function __construct()
    {
        echo 'initiated '.__CLASS__.'<br />';
        $s = &second::getInstance();      
    }
}

class second
{
    static function &getInstance()
    {

        if (!isset($GLOBALS['second'])) 
        {
            $GLOBALS['second'] = new second();
        }
        return $GLOBALS['second'];
    }
    
    function __construct()
    {
        $f = &first::getInstance();
        echo 'initiated '.__CLASS__.'<br />';    
    }
}


$f = &first::getInstance();
$s = &second::getInstance();

PHP:

In bouth cases (using static or $GLOBALS) starts to work normally only when a line with getInstance is commented in one of the classes.

PHP version 5.2.8 WAMP