PHP Session passed to different sub domains

Hi there, Could someone tell me how to pass a session cookie to different sub domains, without passing it in url. For example if I have the following domains: mydomain.com investment.mydomain.com business.mydomain.com market.mydomain.com and want to users to stay logged in while browsing through all this domains. Thanks in advance. Regards, Gera ********************************************************************** Powercor Australia Ltd. This email and any file attachments are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please tell us immediately by return email and delete the document. **********************************************************************