I've seen these, but they still don't answer the original question: why can I not add that to .htaccess without errors?
(I don't have access to php.ini, by the way, and the wp-config edit has never seemed to have any effect.)
It was stated here:
You're host may not have that module set up to be used with .htaccess. What directives (code) are you putting in your .htaccess?
In other words this method will only work if PHP is running as an Apache module with your server. You'll have to refer to your host, as there are quite a few that are not set up like that.