将我的时间转换为时间戳php

我有一个当前的时间格式化方法，它是：

$time = date('mdY');

我需要把它转换回一个普通的linux时间戳。我需要马上做这件事。基本上，一个函数将$time传递给它，然后它需要将其转换为一个正常的时间戳（我假设是time（）；）？

谢谢

编辑：当我尝试每个人明显的建议时，我得到了警告：在/home/content/50/5975650/html/checkEntry.php第75行被零除

function RelativeTime($timeToSend){
    $difference = time() - $timestamp;
    $periods = array("sec", "min", "hour", "day", "week", "month", "years", "decade");
    $lengths = array("60","60","24","7","4.35","12","10");
    if ($difference > 0) { // this was in the past
        $ending = "ago";
    } else { // this was in the future
        $difference = -$difference;
        $ending = "to go";
    }
    for($j = 0; $difference >= $lengths[$j]; $j++)
        $difference /= $lengths[$j];
    $difference = round($difference);
    if($difference != 1) $periods[$j].= "s";
    $text = "$difference $periods[$j] $ending";
    return $text;
}

//检查服务呼叫状态&如果发现未付款，则发送邮件

$query = "SELECT id, account, status, dateEntered FROM service WHERE status = 'Unpaid'";
        $result = mysql_query($query) or die(mysql_error());
    while($row = mysql_fetch_row($result)){
                $account = $row[1];
                $status = $row[2];
                $dateEntered = $row[3];
                $timestamp = strtotime($dateEntered);   
                            $timeToSend = RelativeTime($timestmap);
              // mailStatusUpdate($account, $status, $timeToSend);
        }