mod_rewrite: what does this RewriteRule do?

Given these conditions (I know what they mean/do):

RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d 

What does the first rule do? What is that lonely dash for?

RewriteRule ^.*$ - [NC,开发者_运维知识库L]
RewriteRule ^.*$ /index.php [NC,L]

I've been using this for quite some time now in combination with the Zend Framework, but I never really got what the first rule does exactly.

The RewriteCond directive just describes an additional condition for a RewriteRule directive. So RewriteCond must always be associated with a RewriteRule.

In your case the three RewriteCond probably belong to the first RewriteRule like this:

RewriteCond %{REQUEST_FILENAME} -s [OR]
RewriteCond %{REQUEST_FILENAME} -l [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]

Now this rule is applied if the pattern of the RewriteRule matches the current request URL (per-directory path stripped before) and if the condition is fulfilled.

In this case the condition is only true if when mapping the request URL to the filesystem it matches either an existing file with the file size greater than 0 (-s), or a symbolic link (-l) or a directory (-d). So your rule will be applied for any URL (^.*$ matches anything) that can be mapped to something existing in your filesystem. The substitution - just means to not change anything. And the NC (no case, case insensitive, useless in this context) and L (last rule if applied) are flags that modify either the pattern, replacement or the execution of the rule.