Rand post related question

This is more a generic PHP question, but for what it’s worth:

if ( empty( $_SESSION['seed_time'] ) )
    $_SESSION['seed_time'] = time();

if ( empty( $_SESSION['seed'] ) || time() > ( $_SESSION['seed_time'] + 30 * MINUTE_IN_SECONDS ) ) {
    // Seed session is empty, or session time is older than 30 minutes, generate new seed
    $_SESSION['seed'] = $seed = rand();
    $_SESSION['seed_time'] = time();
} else {
    $seed = $_SESSION['seed'];
}   

return "RAND($seed)";