Request Client Certificate in SSL

[Anon]

Hi guys,
Need a quick hand.
Im coverting an ASP solution to PHP, and am stuck at how to retrieve the clients SSL serial in PHP.
In ASP I do it like..
Request.ServerVariables("CERT_COOKIE")

but im buggered if i can find a way to do it in PHP, im pretty new to PHP AND SSL, so any solution would be great, plus the deadline is Friday.

Thanks in advance...
Baj