How to set a php session cookie to all websites?

[ionideomams]

i'm making a adwords-like to my clients websites only, and i wan't to put the cookies to save the information to all domains (and not just domain \[code\]mydomain.com\[/code\] or subdomains \[code\].mydomain.com\[/code\], i mean all). Code Sample:
\[code\]setcookie('var_name', 'var_value', null, "/", '.mydomain.com' );\[/code\]Can anybody help me?
Thanks