[_ck_]

You don’t want to change that number on every page load, or the CSS file will never ever cache at all and slow down page load / render.

In theory the best number to append would be the timestamp of the CSS file itself, meaning if the CSS file was edited/saved, the number would change, hence force the cache to update. filemtime gives you the file date. The problem with using it is that on some systems it can be relatively slow and add overhead to the page load time.

Alternately you could use today’s date, or even the current hour, ie ?".date('G');

If you are seeing overly aggressive caching of certain files, it probably means the server has been setup that way and can be changed, at least on apache via htaccess