This one was easy - once we knew how to fix it. Seems like the Microsoft full-text search on files and folders ignores files with the ending PHP.

Simply create a .reg file (any filename with the extension "reg" - so it's called filename.reg) and insert the following text into it:

<sub>Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\.php\PersistentHandler]
@=”{5e941d80-bf96-11cd-b579-08002b30bfeb}”</sub>

Then, double-click the file. Confirm all messages - and voila, when you search "files and folders", you'll be able to search for words within PHP files too!