increment date by one month

Use DateTime::add.

$start = new DateTime("2010-12-11", new DateTimeZone("UTC"));
$month_later = clone $start;
$month_later->add(new DateInterval("P1M"));

I used clone because add modifies the original object, which might not be desired.

I needed similar functionality, except for a monthly cycle (plus months, minus 1 day). After searching S.O. for a while, I was able to craft this plug-n-play solution:

function add_months($months, DateTime $dateObject) 
    {
        $next = new DateTime($dateObject->format('Y-m-d'));
        $next->modify('last day of +'.$months.' month');

        if($dateObject->format('d') > $next->format('d')) {
            return $dateObject->diff($next);
        } else {
            return new DateInterval('P'.$months.'M');
        }
    }

function endCycle($d1, $months)
    {
        $date = new DateTime($d1);

        // call second function to add the months
        $newDate = $date->add(add_months($months, $date));

        // goes back 1 day from date, remove if you want same day of month
        $newDate->sub(new DateInterval('P1D')); 

        //formats final date to Y-m-d form
        $dateReturned = $newDate->format('Y-m-d'); 

        return $dateReturned;
    }

Example:

$startDate = '2014-06-03'; // select date in Y-m-d format
$nMonths = 1; // choose how many months you want to move ahead
$final = endCycle($startDate, $nMonths); // output: 2014-07-02

$time = strtotime("2010.12.11");
$final = date("Y-m-d", strtotime("+1 month", $time));

// Finally you will have the date you're looking for.