php caching headers

cache_using_etag.php

function ifNoneMatch($etag, $expire = 60, $cacheAt = "public",$mime_type = 'application/json') {  
  header("Content-Type: $mime_type");
  header("Cache-Control: max-age=$expire, $cacheAt=true");
  header('Expires:' . gmdate('D, d M Y H:i:s T', strtotime("+$expire seconds", $timestamp)));
  header('Etag:' . $etag);

  // exit if not modified
  if ($_SERVER['HTTP_IF_NONE_MATCH'] == $etag) {
    header("HTTP/1.1 304 Not Modified");
    exit;
  }
}

cache_using_modified.php

function ifModifiedSince($timestamp, $expire = 60, $cacheAt = "public",$mime_type = 'application/json') {  
  header("Content-Type: $mime_type");
  header("Cache-Control: max-age=$expire, $cacheAt=true");
  header('Expires:' . gmdate('D, d M Y H:i:s T', strtotime("+$expire seconds", $timestamp)));
  header('Last-Modified:' . gmdate('D, d M Y H:i:s T', $timestamp));

  // exit if not modified
  if (@strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) >= $timestamp) {
    header("HTTP/1.1 304 Not Modified");
    exit;
  }
}