Reliable PHP function to return Monday for week (pre-PHP 7.1)

monday-for-week.php

<?php

/**
* Find the starting Monday for the given week (or for the current week if no date is passed)
*
* This is required as strtotime considers Sunday the first day of a week,
* making strtotime('Monday this week') on a Sunday return the adjacent Monday
* instead of the previous one.
*
* @param string|\DateTime|null $date
* @return \DateTime
*/
function getStartOfWeekDate($date = null)
{
    if ($date instanceof \DateTime) {
        $date = clone $date;
    } else if (!$date) {
        $date = new \DateTime();
    } else {
        $date = new \DateTime($date);
    }
    
    $date->setTime(0, 0, 0);
    
    if ($date->format('N') == 1) {
        // If the date is already a Monday, return it as-is
        return $date;
    } else {
        // Otherwise, return the date of the nearest Monday in the past
        // This includes Sunday in the previous week instead of it being the start of a new week
        return $date->modify('last monday');
    }
}