Find out the error_log's path

An internal server error has often something to do with Apache and /var/log/httpd/ is the error log file of apache, so I think you are in the right file.

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The error path is set in php.ini. To get the path use ini_get():

<?php
  $errorPath = ini_get('error_log');
?>

To only get the path where PHP stores its logs, use:

pathinfo(ini_get('error_log'),PATHINFO_DIRNAME);

Which should return something like this (on localhost):

/Applications/MAMP/logs

Check for error_log setting in php.ini (or in phpinfo())