I had the problem that if there was an error, php would just give me a blank page.  Any error at all forced a blank page instead of any output whatsoever, even though I made sure that I had error_reporting set to E_ALL, display_errors turned on, etc etc.  But simply running the file in a different directory allowed it to show errors!

Turns out that the error_log file in the one directory was full (2.0 Gb).  I erased the file and now errors are displayed normally.  It might also help to turn error logging off.

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