confirming data from PHP script

[activa]

I have a swf that sends data to a PHP script on my server. The script works fine, sends me an email, and sends a string back to the swf. The swf sees the data, but is behaving strangely. Here's what I mean:

function onComplete (event:Event) {
if (event.target != null) {
if (event.target.data == 'secretString') {
trace ('ok');
} else {
trace (event.target.data);
}
}
}

The PHP script sends the correct string. I know because the swf traces 'secretString.' Why doesn't it recognize the string in the if conditional? I've already checked the dataFormat, and it confirms that the data is text.