How do I add exif data to an image?

You can save a large amount of space, especially if you have a large number of images..

Add the following to text.txt (format of the IPTC tags taken from here):

2#110#Credit="My Company"
2#05#Object Name="THE_OBJECT_NAME"
2#55#Date Created="2011-02-03 12:45"
2#80#By-line="BY-LINE?"
2#110#Credit="The CREDIT"
2#115#Source="SOURCE"
2#116#Copyright Notice="THE COPYRIGHT"
2#118#Contact="THE CONTACT"
2#120#Caption="AKA Title"

Strip all existing exif data from the image

mogrify -strip image.jpg

Add the credit to your image

mogrify -profile 8BIMTEXT:text.txt image.jpg

Exiftool looks like it would be an exact match for you.

I haven't tried it but I'm now tempted to go and fix all my honeymoon photos which are marked 01/01/2074 because I forgot to reset the date after the batteries died.

Here's a PHP Exif Library that should do what you need.

The PHP Exif Library (PEL) lets you fully manipulate Exif (Exchangeable Image File Format) data. This is the data that digital cameras place in their images, such as the date and time, shutter speed, ISO value and so on.

Using PEL, one can fully modify the Exif data, meaning that it can be both read and written. Completely new Exif data can also be added to images. PEL is written completely in PHP and depends on nothing except a standard installation of PHP, version 5. PEL is hosted on SourceForge.

on linux there is a program called jhead. It can add a minimal exif header with the command:

jhead -mkexif img.jpg