Bizarrely, PHP files in the deeper, older directory are still working! (i.e. [localhost...] works ok whereas [localhost...] does not - but it does if I move my apache document root back to c:\DIR1\DIR2)
I have changed my %SYSTEMROOT%/php.ini file to the following:
; The root of the PHP pages, used only if nonempty. doc_root = "c:\DIR1"
And I've since turned errors on and off in this file and those changes took effect so it must have read in this change too...
I was trying to access a .php page that didn't exist. I was sure I'd copied my example.php file over but obviously not... strange though because instead of getting a 404 I just got a blank page and the strange PHP error in the Apache Error Log.
I'm really quite embarrassed.... oops.
Can I stop it doing this and make it go back to fiving me a nice 404 if the php page doesn't exist?