As earlier I'd encountered Zends wonderful Zend_Filter class, I decided to go with it and use the Zend_Filter_Encrypt and Zend_Filter_Decrypt to accomplish the work. The Zend_Filter component provides a set of common useful data filters, among which are the encryption filters. Although my project was not developed in Zend, I could easily integrate the required classes in the code. Note that Zend has a great upload library, Zend_File_Transfer, that lets you easily manage file uploading and also encryption, but as I already had the upload code tested, I decided to just add the encryption part.
He includes the step-by-step process to get everything you need and which files you'll need to have included from the framework to make things work. He includes code for both encrypting and decrypting the file information as well as hints on selecting an algorithm and a random initialization vector. You can download the complete source if you want to jump right in.