<?php   /////    etag.php    /////
//  this works for php pages involving only a static file
//     and several other static files that it includes
//      no database dependence is allowed
// $thefiles is an array of names for these files

$hash=""; $browser_hash="";

function makeHash(&$files)
{  $str="";
   foreach($files as $f)
   {   $str .= filemtime($f);   }
   return md5($str);
}

// checks ETag request header
function check_etag(&$files)
{  global $hash, $browser_hash;
   $hash = makeHash($files);
   // $ah = apache_request_headers();
   if ( !empty($_SERVER['HTTP_IF_NONE_MATCH']) )
   {   $browser_hash = $_SERVER['HTTP_IF_NONE_MATCH'];
       if (  strstr($browser_hash, $hash)  )
       {   header("HTTP/1.1 304 Not Modified");
           cache_control_headers();
           exit;
       }
   }
}

function cache_control_headers()
{  global $hash;
   header("Cache-Control: max-age=0, must-revalidate"); 
   header("ETag: \"{$hash}\"");
}

check_etag($thefiles);
// `echo "browser=$browser_hash\n" >> test.txt`
// `echo "computed=$hash\n" >> test.txt`
cache_control_headers();
header("Content-Type: text/html; charset=UTF-8");
?>