My URL is:

http://website.com/folder/blah.php

How do you write a rewrite rule to ensure that the request:

http://website.com/blah.php

Will get quietly rerouted to /folder/ ?

Really?

RewriteRule ^/blah.php /folder/blah.php

does not meet your needs?

See also

http://httpd.apache.org/docs/2.0/mod/mod_rewrite.html#rewriterule

Edit, in line with the comment...

In case your current rule is

RewriteRule ^/(.*)$ /dms/$1

then you definitely should most likely precede it having a condition than will not match when the rule was already applied. For instance

RewriteCond %{REQUEST_URI} !^/dms
RewriteRule ^/(.*)$ /dms/$1

Or else you finish up looping with

blah.php -> /dms/blah.php -> /dms/dms/blah.php -> /dms/dms/dms/blah.php -> etc.

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

RewriteRule ^(.*)$ /folder/$1 [L]