我可以在PHP中使用filemtime（'style.css'）来强制浏览器重新加载css吗？

Can I use filemtime in PHP to force a browser to reload a css?

 $mtime = filemtime('style.css');
 echo "\style.css?mod=" . $mtime;

Is filemtime too slow?

filemtime should be fine under normal circumstances.

You could also look into E-Tags and just have Apache do the work for you.