Therefore in the getInstance() method we cannot use:

$instance = & new Singleton();

We have to use the version without reference:

$instance = new Singleton();

That is ok in most of the cases and whan it becomes problem you can always use a global variable instead of static, but you have to use the superglobal array $GLOBALS to access the global variable, not the global modifier:

function &getInstance() {
    if (!isset($GLOBALS['instance']) || !is_object($GLOBALS['instance']) || !is_a($GLOBALS['instance'], 'Singleton')) {
        $GLOBALS['instance'] =& new Singleton();
    }
    return $instance;
}