But you can set the default mode to PHP/PHP by changing its priority compared to PHP (HTML)'s.
That's how I did it:
- Go to Settings -> Configure Kate -> Open/Save -> Modes & Filetypes;
- Select Scripts/PHP (HTML) Filetype;
- Change Priority to 4;
- Select the File Extensions field and copy it;
- Select Scripts/PHP/PHP Filetype;
- Go to the File Extensions and paste the content copied;
- Hit Apply and Ok;
- Close and reopen Kate.
I was hard to find that solution (see reference); I hope this can be useful.