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');
    }
}

great piece of code...
you save my day buddy!!!

Just AWESOME!
very simple, but does what it must

👍

Thanks mate. Nice snippet.

Thanks!

Thank you so much. You saved my day. May God Bless You always

This is great, thank you, but I think that it can be significantly simplified.
E. g. no need to setTime(): changing time never changes date in PHP.

function getStartOfWeekDate($date = null)
{
    if ($date instanceof \DateTime) {
        $date = clone $date;
    } else {
        // If date is falsy, it won't harm
        $date = new \DateTime($date);
    }
    
    return $date->modify('monday this week');
}

Thanks @abePdIta - it looks like the interpretation of "monday this week" may have been properly fixed from about PHP 7.1 onwards. This snippet was written in 2014 against PHP 5.5 when this was definitely a problem.

Your version, with2020-12-06 as an input (tool):

Output for 5.6.23 - 5.6.40, 7.0.8 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.34, 7.3.0 - 7.3.25, 7.4.0 - 7.4.13, 8.0.0
> 2020-11-30

Output for 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.22, 7.0.0 - 7.0.7
> 2020-12-07

The original, with 2020-12-06 as an input (tool):

Output for 5.3.0 - 5.3.29, 5.4.0 - 5.4.45, 5.5.0 - 5.5.38, 5.6.0 - 5.6.40, 7.0.0 - 7.0.33, 7.1.0 - 7.1.33, 7.2.0 - 7.2.34, 7.3.0 - 7.3.25, 7.4.0 - 7.4.13, 8.0.0
> 2020-11-30

I couldn't pin down one single patch that addresses this issue, but good to know that the "monday this week" behaviour is consistent in actively supported versions of PHP 👍

Regarding setTime(), the week technically starts at midnight on Monday. The original code leveraged this to test for equality between output DateTime objects, but you're correct that it's not strictly required.

Looks like this was tried to be fixed by v5.6.23 and v7.0.8, but only properly fixed by v7.0.17.