new server install question about php and session id

I have just installed apache and php onto a new machine and I've moved my website over.

I think I've made a mistake in the installation because my site users' session id is no longer being tracked in the url as was the case with my old server.

what could be going on here and how do I fix it?

My guess is that there's some setting I've forgotten in php.ini?

Perhaps I'm  off base, maybe it's apache httpd.conf?

hmm...

any insight would be appreciated.