Anyway to dertmine if a user has XMLHTTPRequest capabilities?

Is addition to the Javascript question I posted earlier about detecting if a user has Javascript capabilities, is there anyways to narrow that down even futher to XMLHTTPRequest?

I know there's probably no built in way to do it with PHP, but if there is some sort of clever trick, I'd like to know how to do it. Becuase I know as of this wiriting, Opera (non beta 7.5.4) does not support XMLHTTPRequest, so I'd like to display alternative content for those users as well as others that lack the support.

Note: I know it's possible to emulate it in Opera, but Opera 7.6 is supposed to add in support, and I am mainly looking for backwards compatibility for much older browsers.
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I don't think there is a way, but you could always just use $_SERVER['USER_AGENT'] and have a table of which browsers support it and which dont?

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