database collision?

hi there. i am developing a database app to manage dynamic sites. in a nutshell, i have an item table (to store all the content) and a permission table (to register who's allowed to edit/view specific items). now, when creating a new item, i do the following things: - determine a new permission id (which is the permission table primary key, kinda "SELECT MAX(id) FROM permission_table" and then increase the result by one. i don't use AUTO_INCREMENT columns on purpose.) - create an entry in the permission table - create an entry in the item table, including the permission id as relational attribute now, my question is: since there may be multiple php processes running, if two users simultaneously create an item and post it at the same moment - couldn't it happen that the process of user#1 has already determined the permission id, while user#2 determines the SAME id, creates the entry and user#1 will get an error because the item id was already taken in the meantime? what can i do to avoid such security/integrity holes? thanks for listening - sam