Gets the last Sunday of the month for a given date

last-sunday.php

<?php

/**
 * Gets the last Sunday of the month for a given date.
 *
 * @param string $dateString The input date in 'Y-m-d' format.
 * @return string The last Sunday of the month in 'Y-m-d' format.
 */
function getLastSunday($dateString)
{
    $date = new DateTime($dateString);
    
    // Set the date to the last day of the current month
    $date->modify('last day of this month');

    // Get the day of the week for the last day of the month (0 = Sunday, 1 = Monday, ..., 6 = Saturday)
    $dayOfWeek = $date->format('w');

    // If the last day of the month is not already a Sunday, modify the date to be the last Sunday
    if ($dayOfWeek != 0) {
        $date->modify('last sunday');
    }

    return $date->format('Y-m-d');
}