Drupal - Debugging #AJAX and FormAPI

Using dd($form_state); within the callback function works beautifully.

On linux you can tail -f /tmp/drupal-debug.txt to see the $form_state in real time. (Or whatever the temp directory for your site is set to, often something like sites/yoursite.com/files/tmp.)

Otherwise, you can find drupal-debug.txt in your systems /tmp/ directory. http://ratatosk.net/drupal/tutorials/debugging-drupal.html#dd

The best thing I've found: https://drupal.org/project/devel_debug_log

Part of/depends on Devel and uses ddl to output anything in kpr fashion at admin/reports/debug.

I was also having problems with finding a good way to debug AJAX with the Form API and found debugging with FirePHP to be the best solution:

http://www.thecarneyeffect.co.uk/how-debug-ajax-drupal-using-firephp