<?php
$content_type = "text/css"; // File's content type you're serving dynamically
 
$if_mod = $_SERVER['HTTP_IF_MODIFIED_SINCE']; // Depending on your server, this variable may be differently named
$actual_last_mod = gmdate("U", (filemtime(__FILE__))  );
$if_last_mod_sent = strtotime($if_mod);
 
if (strlen($if_mod) > 2) { // if-last-modified header sent
  if ($actual_last_mod > $if_last_mod_sent) { // Need fresh results
  header("Content-Type: {$content_type}");
  header("Last-modified: " . gmdate("D, d M Y H:i:s", (filemtime(__FILE__))  ) . " GMT" );
  header("Comment: Need fresh results");
  }
  else { // Have latest results
  header("HTTP/1.1 304 Not Modified");
  header("Content-Type: text/html"); // This content-type is always returned for a HTTP 304 response
  header("Comment: Have fresh results");
  exit;
  }
}
else { // No if-last-modified header sent
header("Content-Type: {$content_type}");
header("Last-modified: " . gmdate("D, d M Y H:i:s", (filemtime(__FILE__))  ) . " GMT" );
header("Comment: No if-last-modified header sent");
}
?>