I used Maguma Open Studio (now defunct) for a long time, and although it was pretty buggy, it had a very useful feature that doesn't seem to be in Dev-PHP.
If the cursor was inside the parameter list of a function call (including if you just typed the opening parenthesis) it would pop up a tooltip of the declared parameters. This was very useful, especially since if nothing happened then you knew you hadn't declared it, or forgot to include the file where it was declared. As well as your own functions, it knew about mysql calls etc.
This would be a nice feature to have here.
For the moment, code completion is a static feature.
It doesn't work for user functions, but it should work for MySQL