<minor nitpick>
Your code does not execute some_function() and another_function() every 15 minutes, it puts a 15 minute pause in between, if each function took 5 minutes then it would run them every 25 minutes. To execute them every 15 minutes you would need to check the time() at the top of the loop and again at the bottom and adjust the sleep time accordinly which could still drift your loop by a fraction of a second with each itteration. I do second... errr fourth your suggestion to use cron.
</minor nitpick>>