<?php
$stop_date = '2009-09-30 20:24:00';
echo 'date before day adding: ' . $stop_date;
$stop_date = date('Y-m-d H:i:s', strtotime($stop_date . ' +1 day'));
echo 'date after adding 1 day: ' . $stop_date;
?>
For PHP 5.2.0+, you may also do as follows:
<?php
$stop_date = new DateTime('2009-09-30 20:24:00');
echo 'date before day adding: ' . $stop_date->format('Y-m-d H:i:s');
$stop_date->modify('+1 day');
echo 'date after adding 1 day: ' . $stop_date->format('Y-m-d H:i:s');
?>