Pre-processing of downloadable products.

My installation info is at the bottom of this note.

The bad news is that I'm a complete n00b to Zen Cart. The good news is that I'm a developer of 15 years, so this is just a query for information, not a cry for someone to do everything for me.

The problem is this - my site just had Zen installed as our webstore, and we are going to be selling a lot of Acrobat pdfs. When someone buys a copy of a pdf file, we want checkout to kick off a process that will watermark the pdf with the name of the purchaser. Once that's done, checkout can then continue on with its normal downloadable content process.

I already figured out how to watermark the pdfs - with a command line tool called Coherent. Of course, if there's a better tool more integrated with PHP, mentioning that would be nice. What I'm looking to do with Coherent is to have PHP shell out and run a shell script which runs the watermark command on the outgoing pdf. The info I need is exactly how to go about instrumenting the checkout process so that it does this extra thing with downloadable content during the order processing.

If someone could tell me what the best way to do this in Zen would be, I'd greatly appreciate it. Even to point to a file that I should modify, or the page in the API docs where I should look - anything would help save me days of crawling the net.

Server OS: Linux 2.6.9-67.0.7.ELsmp
Database: MySQL 5.0.77-community
Server Date: 05/30/2009 16:41:56
Database Date: 05/30/2009 13:41:56
Server Up Time: 13:41:56 up 437 days, 5:42, 0 users, load average: 0.11, 0.09, 0.08
HTTP Server: Apache/2.0.63 (Unix) mod_ssl/2.0.63 OpenSSL/0.9.7a mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635
PHP Version: 5.2.6 (Zend: 2.2.0)
PHP Memory Limit: 32M
PHP Safe Mode: Off
PHP File Uploads: On
Max Size: 40M
POST Max Size: 10M
Database Data Size: 225 kB
Database Index Size: 353 kB
Zen Cart 1.3.8a
Database Patch Level: 1.3.8
v1.3.8 [2009-05-25 19:03:46] (Fresh Installation)

Assuming you're using the (safer) download-by-redirect mode that's enabled by default, I would suggest writing an observer class to hook the 'NOTIFY_DOWNLOAD_VIA_SYMLINK___BEGINS' notifier, and use that to intercept the file that's been prepared for download. Perhaps use the symlinked version of the file (maybe add a suffix to the symlink temporarily) as the source for the raw original, then process that to create the new file, wiping out the symlink and creating a new file of the same name in its place. Then the normal download activity will follow.

The logic for preparing and serving the downloads is found in /includes/modules/pages/download/header_php.php
Short of maybe passing a parameter (like the path/filename at hand) to the notifier hook, you shouldn't have to make any alterations to the code in that file when using the observer class to do the added work you need.