Here’s a list of Chart & Graph modules which work with PHP. I personally like PHP/SWF Charts and FusionCharts, which are Flash based charts which you can interface to from PHP. Here’s a list of resources which I checked out.

1. PHP/SWF Charts
2. Fusion Charts
3. Advanced Graph and Chart Collection for PHP
4. RChart PHP Edition
5. ChartDirector
6. JP Graph

Most of the commercial products like PHP/SWF charts and Fusion Charts have a free version also available.