add_action('init', 'start_my_session');
function start_my_session() {
    if (session_status() == PHP_SESSION_NONE) {
        session_start();
    }
 }
add_action('wp_loaded', 'close_my_session', 30);
function close_my_session() {
    if (session_status() == PHP_SESSION_ACTIVE) {
        session_write_close();
    }
 }