Can you check if someone is online on your homepage?

Hi! Can you use a php-script to see if there is someone on your homepage? Im going to make a demo with flash, php,xml and xsl and if more than one at the moment are testing the demo, it won´t work beacause of the work-file is cleaned up every time someone enters.If you can see a digit on the page than you can tell visitor to goto another instance of the page. Thanks for help. Regards Jan