If you're looking at running something that often, it might be time to rethink the design. Firing off a cron-launched PHP process every 15 seconds could be a bit wasteful of resources, especially if a majority of those times it ends up not really having to do anything. But without knowing the whys and wherefores that is bringing this up in the first place, it's hard to be sure.
Perhaps you need to create a daemon process running in the background and awaiting signals or messages to do something. Or perhaps your web app (if it is a web app) could use a bit of code that looks for certain state changes, and when they occur, then they fire off your periodic script, forking it as a background job so that it runs independently at that point. But these are just things to consider, along with other possibilities I've not thought of yet, while looking for the best answer for your specific needs.
