How to maintain session between Domain and subdomain in php

Hi
I am in a problem. I have a site Customermath.com. I have created a new subdomain on comlaint.customermath.com Here any one can come and post there complaint. I am also providing other service on customermath.com which need person to login. I want to maintain session between subdomain like if some one log in through customermath.com then he should not be asked to log in on complaint.customermath.com and vis-versa. Please help me if any one can.

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