I have a senario, where my client operates on many platforms, all that require their users to login and in most cases with different credentials for each platform.
I have suggested a master slave relationship, whereby the master site maintains a database that stores all other platform login details.
So upon logging into the master site, the user can easily login to any of the slave sites, without needing to enter additional login details.
The login details for each slave site would be stored in a database record.
<form method="post" action="http://www.slavedomain.co.uk/index.php" name="myForm" id="myForm">
<input type="hidden" name="username" value="candidate" />
<input type="hidden" name="passwd" value="candidate" />
<!-- now send the form! -->