Dynamic Image Header Optimization

I have a PHP file that displays images that are stored in my database. I'm trying to optimize my website by sending certain headers to the browser that say how long before expiration or when it expires.

I don't want to just use "Cache-Control: max-age" with some time that I estimated the images would expire, because they could either change in 5 months or 1 second from now. But is that the only thing I can do? For each image, I can also get the timestamp of when it was added if that helps at all. What headers would be best to send to the browser?

See if any of these articles help you:

http://devreview.com/php-image-output-a ... r-caching/
http://drupal.org/node/25977
http://www.dynamicdrive.com/forums/arch ... -3685.html
http://www.htaccesselite.com/using-php- ... vt156.html

Yeah... The other day I just ended up just putting a "Last-Modified" header and left it at that.

Alright... glad you got it figured out