Add in Start and End date using PHP

I would like to know how to add in new start and end date using PHP. this is my current coding:

$startdate = date_create(); echo date_format($startdate, 'Y-m-d'); 

$enddate = date_create(); echo date_format($enddate, 'Y-m-d'); 

$actualdate= strtotime('+1 Week', $enddate); 

The end result will always be the current date and not the one that I've insert in. Please help me. Thanks