I do use an external editor now and then for ScriptCase development. However, I develop fully external classes, so that I do not need to be linked in to the ScriptCase environment. When the class is ready I just load it as a library for the project and I can then refer to it in the ScriptCase environment in the various PHP code sections.
For the PHP code sections in a given SC application I usually find the SC editor sufficient as it is often not big sections of code that has to be entered, but occasionally I do copy/paste of these PHP code sections from Rapid PHP (http://www.rapidphpeditor.com/), which I find to be an efficient editor with color coding of key words, code outline, code folding and completion, but not a humongous and resource hungry IDE like Eclipse.
Another good option in Rapid PHP is the preview tab. If one of my applications have errors when generating the application, ScriptCase will display the name of the generated file where the problem is. I then open the generated PHP file and can better see the cause of the problem. When correcting the issue, I can immediately open the preview tab and check/correct until all problems have been solved. The corrected code can then be pasted back into the relevant PHP code section in the ScriptCase environment.
Perhaps it is just me that does not fully know the possibilities of linking an external PHP editor/IDE directly to the ScriptCase environment, because I do not see what benefits one would have from that. The above is for me a good and efficient setup. Rapid PHP is not free, but quite cheap.