Depending upon your needs I'd give this general generic advice:

You can set the SUID or equivalent for the group of the script to allow it to run as another user. I'd try to stay away from this as much as possible, but if you do, ALWAYS use tainting checking to verify your input. If your script is a cgi script, consider writing a go-between script elswhere on the system that is SUID and not viewable directly by the webserver.