500 internal server error on a single node
Display PHP errors for that page or examine your Apache log file with errors enabled. Nobody can tell you what's misconfigured, or whatever typo you've made in your PHP ... log files can tell you though.

EDIT:

For further explanation on "errors" and specifically 500 error from Apache see:
https://stackoverflow.com/questions/8564259/php-5-3-troubleshooting-500-errors-debugging-parse-errors


  PHP will respond with a "500" when it encounters a Fatal error, such
  as E_PARSE, E_ERROR and uncaught exceptions. 500 is the HTTP response
  code for "Internal Server Error" - something unrecoverable happened
  while processing the request.
  
  In your php.ini, on a development machine, you should set your
  error_reporting level to (at least) E_NOTICE - preferably E_ALL - and
  make sure display_errors is on. This will show you the error message
  in the browser, including for "500" errors.
  
  You can also check your Apache error logs, the errors will be listed
  there, as long as log_errors is enabled in php.ini. By default, this
  will be at \logs\error.log.
  
  You can also control all this at run time with ini_set() and
  error_reporting()