The Zend Developer Zone continues their great series of security tips with two new posts - one talking about when to focus on security and the other about file uploads.
From the first, top 17:
Application security should not be a "when all else fails" situation. It's not something you can "put in later". As we've mentioned before, there is no single silver bullet to solve your application security issues. Security is something that should be rolling around in the back of your dead in the design phase, the coding phase, the testing phase, even after you've rolled your code into production.
And, from tip #18:
When you allow users to upload files, your system may be at risk. Always restrict the file types that you allow. Don't rely on a blacklist approach. [...] Be careful with file uploads and make sure you protect them with a whitelist policy instead. Make sure that the file that has been uploaded is of the type that you want to allow.