increment date by one month

Question

Let\'s say I have a date in the following format: 2010-12-11 (year-mon-day)

With PHP, I want to increment the date by one month, and I want the year to be automatica

Answer 1

All presented solutions are not working properly.
strtotime() and DateTime::add or DateTime::modify give sometime invalid results.
Examples:
- 31.08.2019 + 1 month gives 01.10.2019 instead 30.09.2019
- 29.02.2020 + 1 year gives 01.03.2021 instead 28.02.2021
(tested on PHP 5.5, PHP 7.3)

Below is my function based on idea posted by Angelo that solves the problem:

// $time - unix time or date in any format accepted by strtotime() e.g. 2020-02-29  
// $days, $months, $years - values to add
// returns new date in format 2021-02-28
function addTime($time, $days, $months, $years)
{
    // Convert unix time to date format
    if (is_numeric($time))
    $time = date('Y-m-d', $time);

    try
    {
        $date_time = new DateTime($time);
    }
    catch (Exception $e)
    {
        echo $e->getMessage();
        exit;
    }

    if ($days)
    $date_time->add(new DateInterval('P'.$days.'D'));

    // Preserve day number
    if ($months or $years)
    $old_day = $date_time->format('d');

    if ($months)
    $date_time->add(new DateInterval('P'.$months.'M'));

    if ($years)
    $date_time->add(new DateInterval('P'.$years.'Y'));

    // Patch for adding months or years    
    if ($months or $years)
    {
        $new_day = $date_time->format("d");

        // The day is changed - set the last day of the previous month
        if ($old_day != $new_day)
        $date_time->sub(new DateInterval('P'.$new_day.'D'));
    }
    // You can chage returned format here
    return $date_time->format('Y-m-d');
}

Usage examples:

echo addTime('2020-02-29', 0, 0, 1); // add 1 year (result: 2021-02-28)
echo addTime('2019-08-31', 0, 1, 0); // add 1 month (result: 2019-09-30)
echo addTime('2019-03-15', 12, 2, 1); // add 12 days, 2 months, 1 year (result: 2019-09-30)