http://www.phpclasses.org/browse/package/2093.html
and
http://www.phpclasses.org/browse/package/267.html

both of these require the verisign SDK be installed on your server; you'll also need safe mode turned off in order to make system calls. (you'd probably need these 2 conditions to be true with any package that validates/processes with Verisign.) i've used an older version the first package, but it was a little buggy...