PHP - 如何计算添加日期后60天

Let me know :)

$add_date = date ("Y-m-d H:m:s"); 
$expiry_date = 'how?';

How to insert into db the $expiry_date for 60 days. mysql format is datetime

Use strtotime():

$start_date = date('Y-m-d H:m:s');
$end_date = date('Y-m-d H:m:s', strtotime("+60 days"));

or more simply:

$end_date = date('Y-m-d H:m:s', time() + 86400 * 60);

A method avoiding time conversions:

$time = date('Y-m-d H:m:s', time()+3600*24*60)

EDIT
However, it may be less readable and the time saved is probably irrelevant. Plus cletus just edited a similar method into his answer

If you are using PHP >= 5.2 I strongly suggest you use the new DateTime object. For example like below:

$add_date = date("Y-m-d H:m:s"); 
$expiry_date = new DateTime($add_date);
$expiry_date ->modify("+60 days");
echo $expiry_date ->format("Y-m-d H:m:s");

Live Demo