display or redirect to error page

[sde]

it is an association that uses vbulletin. it's not a public forum, so the plugin makes sure the user is cookied with valid credentials from the main assication database.

members are only allowed to have 1 forum account, and that is done by associating their member id in a hidden userfield.

so at global_start, i need to make sure they are a member with a valid cookie, make sure they are logged into the vb account that is associated with their member id, and they can not attempt to register a new user account.

if these conditions fail, i need to display an error message.

currently i have added another php script to display the error, then in the global_start plugin, i use a php header redirect to that page with the error message var in the url.

it works, but i want to make the product so they do not have to upload any files.