php 运算符

function &init_users()
{
$set_modules = false;
static $cls = null;
if ($cls != null)
{
return $cls;
}
include_once(ROOT_PATH . 'includes/modules/integrates/' . $GLOBALS['_CFG']['integrate_code'] . '.php');
$cfg = unserialize($GLOBALS['_CFG']['integrate_config']);
$cls = new $GLOBALS['_CFG']['integrate_code']($cfg);

return $cls;
}

请问init_users()前为什么要加&,加上&是什么意思!