When you do changes to .htaccess files and something is wrong with it you get "Internal server error". That doesn't help me. How can I find out what's the actual error?
Error cause should be saved in server logs. Where it's exactly depends on your server and settings.
Eg. in apache2 log file is defined in ErrorLog in your config or in virtual host.
In the case it is a syntax error, enabling debug may not help you. If you can, run built-in php syntax checker to ensure your file is free from errors:
>> php -l path/to/your/file.php
来源：https://stackoverflow.com/questions/19455322/how-to-debug-internal-server-error