In a source file, how do I prompt guests that they are not allowed to enter

[shuban]

Let's say I made an action=XXX

and the source file XXX.php

In the source file, I want to do a check to see if they are logged in before continuing the code. How would I do that?

[SMFHacks]

Use function disables execution past that point prompts for login
is_not_guest();

[shuban]

Thanks