PHP Shared Sessions across Domain

Question

I have seen a few answers to this on SOF but most of these are concerned with the use of subdomains, of which none have worked for me. The common one being that the use of

Answer 1

1. Define a main session server (I do this in combination with memcached)

2. use Ajax / JSON(P) to request a session from this server, this allows you to share sessions over multiple domains

3. Reuse this session