.htaccess to rewrite one specific file to another specific file

.htaccess to rewrite one specific file to another specific file

All I need to do is rewrite a .js file to a .php file, so when someone
visits "lib/rary.js" they actually get "lib/rary.php".
This is what I'm trying:
RewriteCond ^lib/rary.php lib/rary.js [L]
But it's giving me a server config error.
Any ideas?
Thanks!