Most sites include a number of CSS and JavaScript files. Whilst developing it's usually easier to manage them as separate files but on a live site it makes sense to merge files to reduce the number of HTTP requests the browser has to make. For JavaScript this is particularly important as browsers block rendering whilst downloading. It's also important to version your files to ensure that browsers download the latest copies when you've made changes.

I hate maintaining this stuff manually so I've written a PHP script which takes care of merging files on the fly whilst also versioning the merged file automatically as the various component files change. The file is merged on first request and cached. Subsequent requests are served the cached version. The script also sets HTTP headers to ensure the user's browser maintains each version in its own local cache therefore preventing repeated requests to the server. Finally an archive of the merged files is maintained to ensure that requests for old versions return the relevant CSS/JavaScript rather than the latest which might not match the user's cached HTML.