<?php
/**\
 * @param string $last_modified
 * @param string $etag
 * @return bool
 * @desc original from http://www.nvb.ru/tulz/last~modified~and~etag/
 */
function getIsModified($last_modified = '', $etag = '')
{
    global $_SERVER;
    $refresh = TRUE;

    if ($last_modified == '') $last_modified = time();
    $none_match = (isset($_SERVER['HTTP_IF_NONE_MATCH']))
        ? $_SERVER['HTTP_IF_NONE_MATCH'] : '';
    $modified_since = (isset($_SERVER['HTTP_IF_MODIFIED_SINCE']))
        ? $_SERVER['HTTP_IF_MODIFIED_SINCE'] : '';
    $since = 0;
    if ($modified_since) { // BUG: NetScape sends ";lenght = xxx" after the date
        $arraySince = explode(';', $modified_since);
        $since = strtotime($arraySince[0]);
    }

    switch (TRUE) {
        case (!$none_match && $modified_since):
            if ($since <= time() && is_int($since) && $since >= $last_modified) return FALSE;
            break;
        case ($none_match):
            if ($modified_since) { // Проверка и по If-None-Match, и по If-Modified-Since
                if ($since > time() || !is_int($since) || $since < $last_modified)
                    break; // Файл в кеше клиента устарел по If-Modified-Since
            }
            // Проверку If-Modified-Since, если она была - прошли. Проверка по If-None-Match:
            if ($etag == '') break;
            $INM = split('[,][ ]?', $none_match);
            foreach ($INM as $enity)
                if ($enity == '' || $etag == '' || $enity == '*') return FALSE; // 304 Not Modified
            break;
        default:
            ; // Conditional Get не задан - просто отдаем страницу.
    }
    return TRUE; // Страница изменилась (200 ОК)
}
$is_send_last_modified = false;
function LastModified($last_modified = null) {
    global $is_send_last_modified;
    if (!$last_modified) {
        $last_modified = time();
    }
    $gmt_mtime=gmdate('r', $last_modified);
    if (!$is_send_last_modified) {
        $gmt_mtime = str_replace('+0000', 'GMT', $gmt_mtime);
        header('Last-Modified: ' . $gmt_mtime);
    }
    $is_send_last_modified = true;
    if (isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {
        if (!getIsModified($last_modified)) {
            header('HTTP/1.1 304 Not Modified');
            header('Content-Length: 0');
            exit();
        }
    }
}