Wierd authentication failure

[James]

I'm getting " API Key is required " 500 errors with the following xml:

<request><user>james at liftstudios dot ca</email><password>########</
password></user><api-key>################</api-key></request>

The really strange thing is, the exact same XML works fine from both
the local development version of my (PHP) app, and also from a quick
and dirty ruby script I use to test the api. But when I run the app
that makes this call on our production server, I get the error.

Is there a way of determining why the exact same request would behave
differently when sent from different machines?

thanks,
James

ps I just tried using curl from the command line on each host: works
fine from both. I've also diff'ed the php.ini files to see if there is
some setting on the offending host that is preventing the request from
being sent correctly; nothing there that I can see. I'm using the
HTTP_Request PEAR library to handle the HTTP stuff.