Ok, so you're pretty comfortable with using the Zend Framework, specifically the use of Forms. Along with that, you have a good working knowledge of how to combine a host of standard validators [...] but what do you do when a situation arises that's outside the scope of the pre-packaged validators and filters? Let's say you want to guard against users uploading files that contain viruses, for example. You would have to write a custom validator that checks the uploads aren't infected.

You'll need to install the ClamAV extension on your loal machine for the feature to work. Once it's there, though, it's as simple as setting up the validator (based on Zend_Validate_Abstract) and calling the "cl_scanfile" method on the uploaded file. Also included is the code for a sample controller with a Zend_Form instance and an upload file field to help prove it's working.