File Creation

function XML_Writer($filename, $xml_string)
{
$fh = fopen($filename, 'w')

fwrite($fh, $xml_string);

fclose($fh);
}

No matter what I do I cannot get the debugger to write the file in the function above. I will note that this was working in XP SP2, but I have had to switch over to Vista Ultimate. I'm thinking this is some sort of security issue, but not for sure, has anyone else seen this problem with vs.php for 2008?