PHP DateTime::modify adding and subtracting months

Why it's not a bug:

The current behavior is correct. The following happens internally:

1. +1 month increases the month number (originally 1) by one. This makes the date 2010-02-31.

2. The second month (February) only has 28 days in 2010, so PHP auto-corrects this by just continuing to count days from February 1st. You then end up at March 3rd.

How to get what you want:

To get what you want is by: manually checking the next month. Then add the number of days next month has.

I hope you can yourself code this. I am just giving what-to-do.

PHP 5.3 way:

To obtain the correct behavior, you can use one of the PHP 5.3's new functionality that introduces the relative time stanza first day of. This stanza can be used in combination with next month, fifth month or +8 months to go to the first day of the specified month. Instead of +1 month from what you're doing, you can use this code to get the first day of next month like this:

<?php
$d = new DateTime( '2010-01-31' );
$d->modify( 'first day of next month' );
echo $d->format( 'F' ), "\n";
?>

This script will correctly output February. The following things happen when PHP processes this first day of next month stanza:

1. next month increases the month number (originally 1) by one. This makes the date 2010-02-31.

2. first day of sets the day number to 1, resulting in the date 2010-02-01.

Here is another compact solution entirely using DateTime methods, modifying the object in-place without creating clones.

$dt = new DateTime('2012-01-31');

echo $dt->format('Y-m-d'), PHP_EOL;

$day = $dt->format('j');
$dt->modify('first day of +1 month');
$dt->modify('+' . (min($day, $dt->format('t')) - 1) . ' days');

echo $dt->format('Y-m-d'), PHP_EOL;

It outputs:

2012-01-31
2012-02-29

This may be useful:

echo Date("Y-m-d", strtotime("2013-01-01 +1 Month -1 Day"));
  // 2013-01-31

echo Date("Y-m-d", strtotime("2013-02-01 +1 Month -1 Day"));
  // 2013-02-28

echo Date("Y-m-d", strtotime("2013-03-01 +1 Month -1 Day"));
  // 2013-03-31

echo Date("Y-m-d", strtotime("2013-04-01 +1 Month -1 Day"));
  // 2013-04-30

echo Date("Y-m-d", strtotime("2013-05-01 +1 Month -1 Day"));
  // 2013-05-31

echo Date("Y-m-d", strtotime("2013-06-01 +1 Month -1 Day"));
  // 2013-06-30

echo Date("Y-m-d", strtotime("2013-07-01 +1 Month -1 Day"));
  // 2013-07-31

echo Date("Y-m-d", strtotime("2013-08-01 +1 Month -1 Day"));
  // 2013-08-31

echo Date("Y-m-d", strtotime("2013-09-01 +1 Month -1 Day"));
  // 2013-09-30

echo Date("Y-m-d", strtotime("2013-10-01 +1 Month -1 Day"));
  // 2013-10-31

echo Date("Y-m-d", strtotime("2013-11-01 +1 Month -1 Day"));
  // 2013-11-30

echo Date("Y-m-d", strtotime("2013-12-01 +1 Month -1 Day"));
  // 2013-12-31