adding 1 day to a DATETIME format value

You can use

$now = new DateTime();
$date = $now->modify('+1 day')->format('Y-m-d H:i:s');

You can use as following.

$start_date = date('Y-m-d H:i:s');
$end_date = date("Y-m-d 23:59:59", strtotime('+3 days', strtotime($start_date)));

You can also set days as constant and use like below.

if (!defined('ADD_DAYS')) define('ADD_DAYS','+3 days');
$end_date = date("Y-m-d 23:59:59", strtotime(ADD_DAYS, strtotime($start_date)));

Using server request time to Add days. Working as expected.

25/08/19 => 27/09/19

$timestamp = $_SERVER['REQUEST_TIME'];
$dateNow = date('d/m/y', $timestamp);
$newDate = date('d/m/y', strtotime('+2 day', $timestamp));

Here '+2 days' to add any number of days.

There is a more concise and intuitive way to add days to php date. Don't get me wrong, those php expressions are great, but you always have to google how to treat them. I miss auto-completion facility for that.

Here is how I like to handle those cases:

(new Future(
    new DateTimeFromISO8601String('2014-11-21T06:04:31.321987+00:00'),
    new OneDay()
))
    ->value();

For me, it's way more intuitive and autocompletion works out of the box. No need to google for the solution each time.

As a nice bonus, you don't have to worry about formatting the resulting value, it's already is ISO8601 format.

This is meringue library, there are more examples here.

One liner !

echo (new \DateTime('2016-01-01 +1 day'))->format('Y-m-d H:i:s');