Next pay date

day.php

<?php

function getNextPayDate($day, DateTime $startDate = null)
{

    $date = new DateTime;

    if (null !== $startDate) {
        $date = clone $startDate;
    }

    $nextMonth = $date->format('m') + 1;
    if (13 == $nextMonth) {
        $nextMonth = 1;
    }

    $date->modify("+1 month");

    while ((int)$date->format('m') != $nextMonth) {
        $date->modify('-1 day');
    }

    return $date;
}

$startDate = DateTime::createFromFormat('Y-m-d', '2014-01-31');

$date = getNextPayDate(31, $startDate);
echo $startDate->format('d/m/Y') . ' - ' . $date->format('d/m/Y') . PHP_EOL;