Hi Bill, Aaron, Fedrico and Peter, thanks for your input so far. After reading your mails and some intensive googling, I think the "one custom table for each client" solution is the way to go. These custom tables must be altered by the site administrator whenever a column needs to be added, changed or dropped.
I will extend Zend_Db_Table for my needs to join the custom tables for the current client by default. And I will extend Zend_Form to create elements for the base colums first and the custom client columns afterwards. I think that will be the way to go. Thanks again for your input (special thanks to Bill). Thanks and Best regards, Ralf
