This is a PHP Class useful if you need to resize images keeping their aspect ratio, using the GD Library.

http://www.bitrepository.com/resize-an-image-keeping-its-aspect-ratio-using-php-and-gd.html

How to resize an image (on the fly) and keep its aspect ratio using GD. HotScripts Image Manipulation from Hot Scripts. This is a PHP Class useful if you need to ...

http://www.hotscripts.com/listing/how-to-resize-an-image-on-the-fly-and-keep-its-aspect-ratio-using-gd/

... resizes product shots on-the-fly. ... I wanted the size of the altered image to keep its aspect ratio ... Service Agreements Simplified; Image Manipulation with PHP – The GD ...

http://www.sitepoint.com/image-resizing-php/