Reading through the forums I found out that files (in my case PHP) must be added to a project before the Funtion Tips feature works.

I've done this, however it doesn't work. Function Tips are enabled in Advanced > Configuration > IntelliTip Options. I type a function name then open brackets ... but nothing.

With custom functions (i.e. not PHP inbuilt functions) if I explicitly use Shift-Ctrl-Space the function tip appears.

How can I get this to work automatically for both inbuilt PHP functions AND custom functions?

Add the gl_php.php file from the UEStudio directory to your project - this contains a lot of the php built-ins.