-- or just use -- RewriteRule ^content\.php /content/ [L]
for the second rule if /content/ will resolve to your index file as-is
If the query string is contains id=(something) Then rewrite /content.php?id=something to /content/(something).html and QUIT rewriting because of [L] Otherwise fall through to the next rule, which unconditionally rewrites /content.php to /content/index.html or /content/ if you use the second version
If you do this is .htaccess, this should work as-is. Otherwise you should use ^/content everywhere you see ^content now.