In an htaccess file, mod_rewrite works similar to a <Directory> or <Location> container. and the RewriteBase is used to provide a relative path base.
For example, say you have this folder structure:
root
|-- subdir1
|-- subdir2
|-- subsubdir
So you can access:
http://example.com/ (root)http://example.com/subdir1 (subdir1)http://example.com/subdir2 (subdir2)http://example.com/subdir2/subsubdir (subsubdir)
The URI that gets sent through a RewriteRule is relative to the directory. So if you have:
RewriteRule ^(.*)$ -
in the root, and the request is /a/b/c/d, then the captured URI ($1) is a/b/c/d. But if the rule is in subdir2 and the request is /subdir2/e/f/g then the captured URI is e/f/g. And if the rule is in the subsubdir, and the request is /subdir2/subsubdir/x/y/z, then the captured URI is x/y/z. The directory that the rule is in has that part stripped off of the URI. The rewrite base has no affect on this, this is simply how per-directory works.
What the rewrite base does do, is provide a URL-path base (not a file-path base) for any relative paths in the rule's target. So say you have this rule:
RewriteRule ^foo$ bar.php [L]
The bar.php is a relative path, as opposed to:
RewriteRule ^foo$ /bar.php [L]
where the /bar.php is an absolute path. The absolute path will always be the "root" (in the directory structure above). That means if the rule is in the "root", "subdir1", "subsubdir", etc. The /bar.php path always maps to http://example.com/bar.php.
But the other rule, with the relative path, it's based on the directory that the rule is in. So if
RewriteRule ^foo$ bar.php [L]
is in the "root" and you go to http://example.com/foo, you get served http://example.com/bar.php. But if that rule is in the "subdir1" directory, and you go to http://example.com/subdir1/foo, you get served http://example.com/subdir1/bar.php. etc. This sometimes works and sometimes doesn't, as the documentation says, it's supposed to be required for relative paths, but most of the time it seems to work. Except when you are redirecting (using the R flag, or implicitly because you have http://host in your rule's target). That means this rule:
RewriteRule ^foo$ bar.php [L,R]
if it's in the "subdir2" directory, and you go to http://example.com/subdir2/foo, mod_rewrite will mistake the relative path as a file-path instead of a URL-path and because of the R flag, you'll end up getting redirected to something like: http://example.com/var/www/localhost/htdocs/subdir1. Which is obviously not what you want.
This is where RewriteBase comes in. The directive tells mod_rewrite what to append to the beginning of every relative path. So if I have:
RewriteBase /blah/
RewriteRule ^foo$ bar.php [L]
and that rule is in "subsubdir", going to http://example.com/subdir2/subsubdir/foo will actually serve me http://example.com/blah/bar.php. The "bar.php" is added to the end of the base. In practice, this example is usually not what you want, because you can't have multiple bases in the same directory container or htaccess file.
In most cases, it's used like this:
RewriteBase /subdir1/
RewriteRule ^foo$ bar.php [L]
where those rules would be in the "subdir1" directory and
RewriteBase /subdir2/subsubdir/
RewriteRule ^foo$ bar.php [L]
would be in the "subsubdir" directory.
This partly allows you to make your rules portable, so you can drop them in any directory and only need to change the base instead of a bunch of rules. For example if you had:
RewriteEngine On
RewriteRule ^foo$ /subdir1/bar.php [L]
RewriteRule ^blah1$ /subdir1/blah.php?id=1 [L]
RewriteRule ^blah2$ /subdir1/blah2.php [L]
...
such that going to http://example.com/subdir1/foo will serve http://example.com/subdir1/bar.php etc. And say you decided to move all of those files and rules to the "subsubdir" directory. Instead of changing every instance of /subdir1/ to /subdir2/subsubdir/, you could have just had a base:
RewriteEngine On
RewriteBase /subdir1/
RewriteRule ^foo$ bar.php [L]
RewriteRule ^blah1$ blah.php?id=1 [L]
RewriteRule ^blah2$ blah2.php [L]
...
And then when you needed to move those files and the rules to another directory, just change the base:
RewriteBase /subdir2/subsubdir/
and that's it.
