Как я должен реализовать ленивое создание сессии в PHP?

Я разработал рабочее решение этой проблемы, которое использует session_set_save_handler() и набор пользовательских методов хранения сессии, которые проверяют наличие содержимого в массиве $_SESSION перед записью данных сессии. Если для сессии нет данных для записи, то header('Set-Cookie:', true); используется для предотвращения отправки сессионного cookie PHP в ответ.

Последняя версия этого кода, а также документация и примеры доступны на GitHub. В приведенном ниже коде важными функциями, которые заставляют это работать, являются lazysess_read($id) и lazysess_write($id, $sess_data).

<?php
/**
 * This file registers session save handlers so that sessions are not created if no data
 * has been added to the $_SESSION array.
 * 
 * This code is based on the session handling code in Pressflow (a backport of
 * Drupal 7 performance features to Drupal 6) as well as the example code described
 * the PHP.net documentation for session_set_save_handler(). The actual session data
 * storage in the file-system is directly from the PHP.net example while the switching
 * based on session data presence is merged in from Pressflow's includes/session.inc
 *
 * Links:
 *      http://www.php.net/manual/en/function.session-set-save-handler.php
 *      http://bazaar.launchpad.net/~pressflow/pressflow/6/annotate/head:/includes/session.inc
 *
 * Caveats:
 *      - Requires output buffering before session_write_close(). If content is 
 *        sent before shutdown or session_write_close() is called manually, then 
 *        the check for an empty session won't happen and Set-Cookie headers will
 *        get sent.
 *        
 *        Work-around: Call session_write_close() before using flush();
 *        
 *      - The current implementation blows away all Set-Cookie headers if the
 *        session is empty. This basic implementation will prevent any additional
 *        cookie use and should be improved if using non-session cookies.
 *
 * @copyright Copyright &copy; 2010, Middlebury College
 * @license http://www.gnu.org/copyleft/gpl.html GNU General Public License (GPL), Version 3 or later.
 */ 

/*********************************************************
 * Storage Callbacks
 *********************************************************/

function lazysess_open($save_path, $session_name)
{
    global $sess_save_path;

    $sess_save_path = $save_path;
    return(true);
}

function lazysess_close()
{
    return(true);
}

function lazysess_read($id)
{ 
    // Write and Close handlers are called after destructing objects
    // since PHP 5.0.5.
    // Thus destructors can use sessions but session handler can't use objects.
    // So we are moving session closure before destructing objects.
    register_shutdown_function('session_write_close');

    // Handle the case of first time visitors and clients that don't store cookies (eg. web crawlers).
    if (!isset($_COOKIE[session_name()])) {
        return '';
    }

    // Continue with reading.
    global $sess_save_path;

    $sess_file = "$sess_save_path/sess_$id";
    return (string) @file_get_contents($sess_file);
}

function lazysess_write($id, $sess_data)
{ 
    // If saving of session data is disabled, or if a new empty anonymous session
    // has been started, do nothing. This keeps anonymous users, including
    // crawlers, out of the session table, unless they actually have something
    // stored in $_SESSION.
    if (empty($_COOKIE[session_name()]) && empty($sess_data)) {

        // Ensure that the client doesn't store the session cookie as it is worthless
        lazysess_remove_session_cookie_header();

        return TRUE;
    }

    // Continue with storage
    global $sess_save_path;

    $sess_file = "$sess_save_path/sess_$id";
    if ($fp = @fopen($sess_file, "w")) {
        $return = fwrite($fp, $sess_data);
        fclose($fp);
        return $return;
    } else {
        return(false);
    }

}

function lazysess_destroy($id)
{
    // If the session ID being destroyed is the one of the current user,
    // clean-up his/her session data and cookie.
    if ($id == session_id()) {
        global $user;

        // Reset $_SESSION and $user to prevent a new session from being started
        // in drupal_session_commit()
        $_SESSION = array();

        // Unset the session cookie.
        lazysess_set_delete_cookie_header();
        if (isset($_COOKIE[session_name()])) {
            unset($_COOKIE[session_name()]);
        }
    }


    // Continue with destruction
    global $sess_save_path;

    $sess_file = "$sess_save_path/sess_$id";
    return(@unlink($sess_file));
}

function lazysess_gc($maxlifetime)
{
    global $sess_save_path;

    foreach (glob("$sess_save_path/sess_*") as $filename) {
        if (filemtime($filename) + $maxlifetime < time()) {
            @unlink($filename);
        }
    }
    return true;
}

/*********************************************************
 * Helper functions
 *********************************************************/

function lazysess_set_delete_cookie_header() {
    $params = session_get_cookie_params();

    if (version_compare(PHP_VERSION, '5.2.0') === 1) {
        setcookie(session_name(), '', $_SERVER['REQUEST_TIME'] - 3600, $params['path'], $params['domain'], $params['secure'], $params['httponly']);
    }
    else {
        setcookie(session_name(), '', $_SERVER['REQUEST_TIME'] - 3600, $params['path'], $params['domain'], $params['secure']);          
    }
}

function lazysess_remove_session_cookie_header () {
    // Note: this implementation will blow away all Set-Cookie headers, not just
    // those for the session cookie. If your app uses other cookies, reimplement
    // this function.
    header('Set-Cookie:', true);
}

/*********************************************************
 * Register the save handlers
 *********************************************************/

session_set_save_handler('lazysess_open', 'lazysess_close', 'lazysess_read', 'lazysess_write', 'lazysess_destroy', 'lazysess_gc');

Хотя это решение работает и в основном прозрачно для приложений, включающих его, оно требует переписывания всего механизма хранения сессий, а не полагаться на встроенные механизмы хранения с переключателем сохранять или нет.