• In tip #10, Cal looks briefly at some of the dangers of blindly using form input when sending a mail. One never knows what kind of nasty headers a user might enter.
• Tip #11 recommends the "path of least privileges" when it comes to allowing access to your application. Don't go global when simple will do just fine - even with the best of intentions, the wrong access can lead to big issues.
• Finally, in tip #12, one of the more discouraged functions in PHP is discussed - eval. This one little function, when fed the wrong kind of string, can unravel your application from the inside out and provide a would-be attacker just the opening they might need.

You can check out more great security tips like these on the Zend Developer Zone website.