The Zend Developer Zone has posted two more handy security tips - one concerning file/directory naming and the other about input validation.
From the first tip:
Don't rely on obscure names to keep your application safe. You should always check permissions, test for vulnerabilities with testing tools and keep an eye on your log files for suspicious activity. When designing your applications and web sites though, don't make it easy for bad people to do bad things. Donâ€™t use default or common names for your files and directories.
And from the second:
It's a sad fact of life but users are evil. Users want nothing more than to find a way to exploit your application. As soon as you let your guard down and start thinking "I'm only selling small stuffed animals so how evil can my users really be?" you've lost the battle.
You can catch up on these and the other previous security tips on this page on the Zend Developer Zone website.