Obtaining the next month in PHP

Over and over again PHP users complain that next month in PHP's date-string parser doesn't go to the next month, but instead skips to the one after next month; like in the following example:

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

The output of the little script will be March. March obviously doesn't follow January as February is in between. However, the current behavior is correct. The following happens internally:

• next month increases the month number (originally 1) by one. This makes the date 2010-02-31.
  
• 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.
  
• The formatting strips off the year and day, resulting in the output March.
  

This can easily be seen when echoing the date with a full date format, which will output March 3rd, 2010:

<?php
echo $d->format( 'F jS, Y' ), "\n";
?>

To obtain the correct behavior, you can use some of 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 next month from the previous example, we use first day of next month here:

<?php
$d = new DateTime( '2010-01-08' );
$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:

• next month increases the month number (originally 1) by one. This makes the date 2010-02-31.
  
• first day of sets the day number to 1, resulting in the date 2010-02-01.
  
• The formatting strips off the year and day, resulting in the output February.
  

Besides first day of, there is an equivalent last day of to go to the last day of a month. The following example demonstrates this:

<?php
$d = new DateTime( '2010-01-08' );
$d->modify( 'last day of next month' );
echo $d->format( 'F jS, Y' ), "\n";
?>

This outputs February 28th, 2010. Internally the following happens:

• next month increases the month number (originally 1) by one. This makes the date 2010-02-08.
  
• last day of increases the month number by one, and sets the day number to 0, resulting in the date 2010-03-00.
  
• PHP then auto-corrects the invalid day number 0 by removing one from the month and skipping to the last day of that month, resulting in 2010-02-28.