Authentication through password or certificate

Hi all,

i was wondering if there is a possibility to authenticate users through
certificates. I know it's possible with apache, but because i work with
RewriteRules, i can't authenticate on directories.
I know it's possible with php to use the header() to do the basic
authentication, but i was wondering if it was possible to FIRST look for a
ssl certificate, and if it's not there, or not signed, show the basic
authentication.

regards,