gistfile1.php

<?php

/**
 * @return DateTime
 */
function getNextPaymentDate(DateTime $lastPaymentDate = null)
{
    // 1. Get start date
    $day = 1;

    // 2. Get now
    $now = new DateTime();

    // 3. Get current day, month, year
    $currentDay = $now->format('d');
    $currentMonth = $now->format('m');
    $currentYear = $now->format('Y');
    var_dump($now);
    if ($currentDay > 1) { // 4. If payment day has already gone, we should take next month
        $now->modify('+1 month');
        $month = $now->format('m');
        $year = $now->format('Y');
        var_dump($now);
        // 5. If due date is today and payment has been made today, we should move to next month
    } elseif (($currentDay == 1)
        && $lastPaymentDate
        && $lastPaymentDate->format('Ymd') == $now->format('Ymd')
    ) {
        $now->modify('+1 month');
        $month = $now->format('m');
        $year = $now->format('Y');
    } else { // 5. If the day is Ok, month and year should be the current ones
        $month = $currentMonth;
        $year = $currentYear;
    }

    $daysInMont = cal_days_in_month(CAL_GREGORIAN, $month, $year);
    if ($day > $daysInMont) {
        $day = $daysInMont;
    }
    var_dump($year);
    var_dump($month);
    var_dump($day);
    exit;
    return new DateTime(implode('-', array($year, $month, $day)));
}

$date = DateTime::createFromFormat('Y-m-d', '2014-02-28');

$date2 = getNextPaymentDate($date);
var_dump($date2);