works in 4.4.2 - breaks in 5.2.1

["Jim Berkey" <jim@xxxxxxxx>]

I'm familiar with actionscript, but pretty new to php . . .I have a guestbook script that worked fine until my host upgraded to php 5.2.1 - can anyone tell me what part of my if statement or variable is breaking in 5.2.1? I send the new data with Flash using xml.sendAndLoad, to the xml file via standalone php file shown below.
tia,
jimbo

<?php
$file = fopen("guestbook.xml", "w+") or die("Can't open XML file");
$xmlString = $HTTP_RAW_POST_DATA;
if(!fwrite($file, $xmlString)){
    print "Error writing to XML-file";
}
print $xmlString."\n\n";
fclose($file);

exit;
?>