php - hash css and js files to break cache. Is it slow?

Question

I have some script that generates templates of a page. Also, this scripts renders <script> and <link rel='stylesheet'> tags in the HTML.

I'd like to add cache-breaking feature with "?v=xxxxx" parameter.

I do it in such a way:

foreach ($scripts as &$script) {

    // get script file name
    $script = "{$this->_js_folder}/{$script}";

    // get it's realpath
    $realfile = realpath(substr($script,1));

    // hashing the file
    $hash = md5_file($realfile);

    // adding cache-breaking number
    $script .= '?v='.$hash;

} //: foreach

Isn't it slow, to hash about a dozen files every time user refreshes the page?

Answer 1

Personally, I wouldn't hash the file, that's a waste of resources. Instead of it, i would add the last-modified timestamp into the v?=.... I mean something like this:

foreach ($scripts as &$script) {

    // get script file name
    $script = "{$this->_js_folder}/{$script}";

    // get it's realpath
    $realfile = realpath(substr($script,1));

    // getting last modified timestamp
    $timestamp = filemtime($realfile);

    // adding cache-breaking number
    $script .= '?v='.$timestamp;

} //: foreach