That cron you have right now (with the 5 *'s) will run your job every minute.
If you want every 15 minutes, you should do as mcavic suggested, and use
And to prevent output completely, your cron should look like:
You won't get notifications of failures, though. If you want those, use nalin's redirect method.
That remote script you're running, you're not actually executing PHP on it, correct? You're just getting some HTML or XML back from the page on the remote server? If that's the case, then using wget should be fine as well, since you're not actually executing the PHP binary on your hosts' systems.