If you need to cache static content in one of your sub folders use the following snippet of code placed into an htaccess file.
What this code will do is cache the content for 2 days (172800 seconds) and not request the content again unless it has changed.
Additionally as per the comments, it will only cache PHP files if they specifically send their own cache headers.
FileETag MTime Size
# Enable expirations.
# Cache all files for 2 weeks after access (A).
# Do not allow PHP scripts to be cached unless they explicitly send cache
# headers themselves. Otherwise all scripts would have to overwrite the
# headers set by mod_expires if they want another caching behavior.