While I think this is a very valuable resource, and patterns are always welcome by me, it should be noted that the Apache docs recommend against using .htaccess files due to the performance penalty.
You should avoid using .htaccess files completely if you have access to httpd main server config file. Using .htaccess files slows down your Apache http server. Any directive that you can include in a .htaccess file is better set in a Directory block, as it will have the same effect with better performance.
PSA: Authorization snippets are based on Apache 2.2 config syntax. Care should be taken to review, update and test Authorization config when upgrading to Apache 2.4 (http://httpd.apache.org/docs/2.4/upgrading.html).
The most important thing when it comes to web server configuration is to have a testing setup you understand and that is fully under you're control. That goes twice for working with mod_rewrite.
My recommendation: use only curl -I so caching is ruled out as a problem source. Use a virtual machine that you can reprovision quickly and reliably. Crank up the log level to debug in you apache config. And don't give up!
Actually, I dont' think that is the webserver's job.
Relying on your webserver to protect you against SQL injection is probably not what you want to do. The webserver has no knowledge at all about what kind of program you run behind it. You would need to teach it everything about what you're doing.
Seriously, you are much better off just using prepared statements everywhere than trying to teach a webserver the finer points of your particular combination of SQL and the language you use. It's like parsing HTML with regular expressions. It might hold up for a while or for certain tasks, but will explode quite unexpectedly at some later point.
That's true, but sometimes (especially with completely naive or old PHP) 'using prepared statements everywhere' means 'rewriting everything.' In those cases, htaccess might be the only flexible option you have until you can.
From the docs (http://httpd.apache.org/docs/current/howto/htaccess.html):
You should avoid using .htaccess files completely if you have access to httpd main server config file. Using .htaccess files slows down your Apache http server. Any directive that you can include in a .htaccess file is better set in a Directory block, as it will have the same effect with better performance.