Example safe use of eval()

I am trying to get code that I have saved as a string in a database to evaluate as code within a larger script with a loop. There are obvious security concerns with allowing users to input code into the field if it is not preprocessed before it is evaluated. The eval function appears to be the ticket with PHP 4 since it will return the results as well. Has anyone got an example of an appropriate preprocessing strategy and/or example of eval() being used this way? Thanks to those that replied to my original post. -- Dave Pratt