Explain .htaccess rules. Doesn't work for me - apache

Question

I have hierarchy structure of file as below:
/public_html/directory/
/public_html/directory/index.php
/public_html/directory/file.php
I want to forward the request to the most appropriate link. For example,
mydomain.com/directory/ to mydomain.com/directory/index.php
mydomain.com/directory/file to mydomain.com/directory/file.php
If 404 or 500 error occurs then forward it to /public_html/error.php
For this I have written following code in .htaccess
Options +SymLinksIfOwnerMatch -MultiViews
# Turn mod_rewrite on
RewriteEngine On
RewriteBase /
# don't touch /forum URIs
RewriteRule ^forums/ - [L,NC]
# hide .php extension snippet
#To externally redirect /dir/foo.php to /dir/foo
RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s([^.]+).php [NC]
RewriteRule ^ %1 [R,L]
# To internally forward /dir/foo to /dir/foo.php
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.*?)/?$ $1.php [L]
ErrorDocument 404 /error.php
ErrorDocument 500 /error.php
It works properly if I make requests as mentioned above. But if I request for page mydomain.com/directory/file/scrap then it generates Internal Server Error
I think its because file is not a directory. But I don't know how to solve this problem here. Please suggest.
Also one more line is mentioned in the error output by server regarding ErrorDocument for 500 in the end. The complete output is as follows:
Internal Server Error
The server encountered an internal error or misconfiguration and was
unable to complete your request.
Please contact the server administrator, root#localhost and inform
them of the time the error occurred, and anything you might have done
that may have caused the error.
More information about this error may be available in the server error
log.
Additionally, a 500 Internal Server Error error was encountered while
trying to use an ErrorDocument to handle the request.

Answer 1

This is happening because the %{REQUEST_FILENAME} -f condition also checks for "path info" style requests. So when you request:
mydomain.com/directory/file/scrap
The rewrite engine will see the file /directory/file.php and assume that you're making a path info style request, e.g. /directory/file.php/scrap
This causes the rewrite engine to loop.
You need to change that condition to:
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI}.php -f
Oh, also, as for the ErrorDocument thing, I'm pretty sure that what's happening is the rewrite module is completely ending the URL processing in the pipeline. That means, directives that get applied at the end of the pipeline will never get applied if mod_rewrite flips out and ends all the processing. That's why your ErrorDocument directives aren't being applied. If you fix the rewrite issue, you'll notice that the 500 error document will work again.

Answer 2

Based on the configuration above, I would say there is a problem with /error.php.
Assuming there is no scrap.php, mydomain.com/directory/file/scrap will not get rewritten.
The 500 error also mentions that the error was encountered while trying to use an ErrorDocument.
Try a static error.html instead of a dynamic error.php to narrow the problem down.
You could also log the rewrites to verify that they are working correctly:
RewriteLogLevel 9
RewriteLog /var/log/httpd/rewrite_log