Nginx empty error log

I am trying to install piwik

The system check during installation shows a 500 error in one of the tests.

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However the error file in the log directory is empty while the access file states the correct value

Code:

 "GET /piwik.php?url=http://example.com HTTP/1.1" 500 5

Why doesn't the error appear in the error log file to troubleshoot the problem?

 

ok, and how do I figure out the problem that PHP reported?

I can't find the correct logfile anywhere...

 

The file I was looking for is located here:

Code:

tail -f /var/log/php-fpm/www-php.error.log