With php 5.3
$date = new DateTime();
$interval = new DateInterval('P1D');
echo $date->format('Y-m-d') , PHP_EOL;
$date->add($interval);
echo $date->format('Y-m-d'), PHP_EOL;
$date->add($interval);
echo $date->format('Y-m-d'), PHP_EOL;
will output
2012-12-24
2012-12-25
2012-12-26