I WAS inconsistently using PHP end tags.
Got white screen on FF and 400 or 500 error (can't remember which) on Chrome.
Stumbled onto inconsistent use of PHP end tags as the possible problem by googling.
Now have policy for using PHP end tags: no PHP end tags unless file contains HTML.
Problem fixed except I don't know for a fact, that inconsistent use of PHP tags caused the problem
How likely is it that an nconsistent use of PHP tags cause white screen / server error?
Is no PHP end tags unless file contains HTML a good policy?
If so, why?