PHP & 'Fixed entry' based arguments not working

It looks like PHP arguments within the Views Or argument block aren't being used at all. I've configured mine with 'Provide default argument' -> PHP code -> return 'test'; but the query always ignores it.

The same happens with 'Fixed entry' in the 'Provide default argument' options.

The arguments work fine if entered in the Arguments test field when building the view and I assume they would work if entered in the url. Its just the default arguments that aren't.

Any clues on how to fix this?