<p>I need to implement a SSO between a SharePoint site and a custom PHP-based site which resides on a different domain. I need to use the SharePoint user accounts to log in the PHP site. (I'll have a full control over the PHP source)</p>
<p>Is this possible? Any tips and tricks?</p>

I don't know much about this area but hopefully this might help point you in the right direction.
Investigate LDAP...
You can set up PHP to use LDAP credentials. If your SharePoint site uses Active Directory, then you can expose this directory as an LDAP source and use that in the PHP application.
Automated sign-in...
Having the sign in happen automatically between each site is a very different matter. e.g. I'm logged into MOSS already, click on a link that goes to the PHP app and find that I'm already logged in there as well. For this you will need to investigate using something like Kerberos keys/authentication. It's a messy and difficult area.