g1smd - 4:40 pm on Jan 17, 2013 (gmt 0)
I said the PHP file can be as large as you like.
If you rewrite underscore requests to a PHP script, then that's one line of code in the htaccess file.
RewriteRule _ /handlethis.php [L]
The PHP file does the real work, looking up the new URL that replaces the old, and sending the correct HTTP headers (301 or 404) back to the browser.
You can have thousands of lines in htaccess if you want, but the code becomes unmanageable. There are better solutions.