How to get next day, last day and next monday like dates in PHP

<?php
function formatNextMondayFromNow( DateTime $dt )
{
        return $dt->modify( 'next monday' )->format( 'Y-m-d' );
}

$d = new DateTime();
echo formatNextMondayFromNow( $d ), "\n";
echo $d->format( 'Y-m-d' ), "\n";

 echo "<br>--------------------------------------------<br>";
?>

<?php
function formatNextdayFromNow( DateTime $dt )
{
        return $dt->modify( 'next day' )->format( 'Y-m-d' );
}

$d = new DateTime();
echo formatNextdayFromNow( $d ), "\n";
echo $d->format( 'Y-m-d' ), "\n";

 echo "<br>--------------------------------------------<br>";
?>

<?php
function formatLastMondayFromNow( DateTime $dt )
{
        return $dt->modify( 'last monday' )->format( 'Y-m-d' );
}

$d = new DateTime();
echo formatNextMondayFromNow( $d ), "\n";
echo $d->format( 'Y-m-d' ), "\n";

 echo "<br>--------------------------------------------<br>";
?>

<?php
function formatLastdayFromNow( DateTime $dt )
{
        return $dt->modify( 'last day' )->format( 'Y-m-d' );
}

$d = new DateTime();
echo formatLastdayFromNow( $d ), "\n";
echo $d->format( 'Y-m-d' ), "\n";

 echo "<br>--------------------------------------------<br>";
?> 
Some brief explanation and it is very important.

The first time that my improved DateTime support made its way into PHP was officially in PHP 5.1, although the more advanced features such as the DateTime class only made it appearance in PHP 5.2. Since its introduction the DateTime class implementation suffered from one design mistake — arguably not something that even an RFC would have highlighted.
In PHP, if you do the following:

<?php
function formatNextMondayFromNow( DateTime $dt )
{
        return $dt->modify( 'next monday' )->format( 'Y-m-d' );
}

$d = new DateTime();
echo formatNextMondayFromNow( $d ), "\n";
echo $d->format( 'Y-m-d' ), "\n";
?>

It displays:

2014-02-17
2014-02-17

The modify() method does not only return the modified DateTime object, but also changes the DateTime object it was called on. In an API like above, this is of course totally unexpected and the only way to avoid this behaviour is to do the following instead:

echo formatNextMondayFromNow( clone $d ), "\n"; 

This mutability property that all modifying methods of the DateTime class have is highly annoying, and something that I would now rather remove. But of course we cannot as that would break backwards compatibility.

So in PHP 5.5, after a few stumbles, I finally managed to rectify this. I did not change the original class's behaviour, but instead, I added a new class. The new DateTimeImmutable class which does not display this "mutable" behaviour, and only returns the modified object:
<?php 
function formatNextMondayFromNow( DateTimeImmutable $dt ) { 
 return $dt->modify( 'next monday' )->format( 'Y-m-d' ); 
}
$d = new DateTimeImmutable();
 echo formatNextMondayFromNow( $d ), "<br>";
 echo $d->format( 'Y-m-d' ), "\n"; 
?>