Wordpress users can now enable autocomplete for >1700 wordpress functions via the site settings window.
This is available now in the dev channel release. You can switch to dev channel from:
You can also add your own custom definitions. To do this upload a json file to your web server and place the url in the custom completions field.
To generate completions from a php file. Open the file in ShiftEdit, then open chrome's js console (ctrl+shift+j).
Then copy and paste the output (between the quotes) into your json file.