does anyone know how to tell if a PHP scrip is being executed from the command line not a server HTTP request??

I am trying to write a script but for security I need to verify that it is not executed by a web server. At this point I have just placed it outside the htdocs tree but would like to build into the script a test to die if it is being executed by a web server. Thanks Jeff Means CIO for MeansPC meaje@meanspc.com