Internal Error 500 Apache, but nothing in the logs?

I just ran into this and it was due to a mod_authnz_ldap misconfiguration in my .htaccess file. Absolutely nothing was being logged, but I kept getting a 500 error.

If you run into this particular issue, you can change the log level of mod_authnz_ldap like so:

LogLevel warn authnz_ldap_module:debug

That will use a log level of debug for mod_authnz_ldap but warn for everything else (https://httpd.apache.org/docs/2.4/en/mod/core.html#loglevel).

Why are the 500 Internal Server Errors not being logged into your apache error logs?

The errors that cause your 500 Internal Server Error are coming from a PHP module. By default, PHP does NOT log these errors. Reason being you want web requests go as fast as physically possible and it's a security hazard to log errors to screen where attackers can observe them.

These instructions to enable Internal Server Error Logging are for Ubuntu 12.10 with PHP 5.3.10 and Apache/2.2.22.

Make sure PHP logging is turned on:

1. Locate your php.ini file:

   el@apollo:~$ locate php.ini
   /etc/php5/apache2/php.ini
   

2. Edit that file as root:

   sudo vi /etc/php5/apache2/php.ini
   

3. Find this line in php.ini:

   display_errors = Off
   

4. Change the above line to this:

   display_errors = On
   

5. Lower down in the file you'll see this:

   ;display_startup_errors
   ;   Default Value: Off
   ;   Development Value: On
   ;   Production Value: Off
   
   ;error_reporting
   ;   Default Value: E_ALL & ~E_NOTICE
   ;   Development Value: E_ALL | E_STRICT
   ;   Production Value: E_ALL & ~E_DEPRECATED
   

6. The semicolons are comments, that means the lines don't take effect. Change those lines so they look like this:

   display_startup_errors = On
   ;   Default Value: Off
   ;   Development Value: On
   ;   Production Value: Off
   
   error_reporting = E_ALL
   ;   Default Value: E_ALL & ~E_NOTICE
   ;   Development Value: E_ALL | E_STRICT
   ;   Production Value: E_ALL & ~E_DEPRECATED
   

   What this communicates to PHP is that we want to log all these errors. Warning, there will be a large performance hit, so you don't want this enabled on production because logging takes work and work takes time, time costs money.

7. Restarting PHP and Apache should apply the change.

8. Do what you did to cause the 500 Internal Server error again, and check the log:

   tail -f /var/log/apache2/error.log
   

9. You should see the 500 error at the end, something like this:

   [Wed Dec 11 01:00:40 2013] [error] [client 192.168.11.11] PHP Fatal error:  
   Call to undefined function Foobar\\byob\\penguin\\alert() in /yourproject/
   your_src/symfony/Controller/MessedUpController.php on line 249
   

Check your php error log which might be a separate file from your apache error log.

Find it by going to phpinfo() and check for error_log attribute. If it is not set. Set it: https://stackoverflow.com/a/12835262/445131

Maybe your post_max_size is too small for what you're trying to post, or one of the other max memory settings is too low.