php返回相对时间，如：20分钟前，3天前

function plural($num) {
    if ($num != 1)
        return "s";
}

function getRelativeTime($date) {
    $diff = time() - strtotime($date);
    if ($diff<60)
        return $diff . " second" . plural($diff) . " ago";
    $diff = round($diff/60);
    if ($diff<60)
        return $diff . " minute" . plural($diff) . " ago";
    $diff = round($diff/60);
    if ($diff<24)
        return $diff . " hour" . plural($diff) . " ago";
    $diff = round($diff/24);
    if ($diff<7)
        return $diff . " day" . plural($diff) . " ago";
    $diff = round($diff/7);
    if ($diff<4)
        return $diff . " week" . plural($diff) . " ago";
    return "on " . date("F j, Y", strtotime($date));
}