Sorry, I forgot to change 2 & 3 

Expected result:
----------------
1 2010/02/20
2 2010/02/01
3 2010/02/28
4 2010/03/20
5 2010/03/01
6 2010/03/31

That's because those things were introduced in PHP 5.3:

derick@kossu:~$ pe 5.2dev
derick@kossu:~$ php
<?php
echo strftime('1 %Y/%m/%d')."\n";
echo strftime('2 %Y/%m/%d', strtotime('first day'))."\n";
echo strftime('3 %Y/%m/%d', strtotime('last day'))."\n";
echo strftime('4 %Y/%m/%d', strtotime('next month'))."\n";
echo strftime('5 %Y/%m/%d', strtotime('first day next month'))."\n";
echo strftime('6 %Y/%m/%d', strtotime('last day next month'))."\n";
?>
1 2010/02/20
2 2010/02/21
3 2010/02/19
4 2010/03/20
5 2010/03/21
6 2010/03/19


derick@kossu:~$ pe 5.3dev
derick@kossu:~$ php
<?php
echo strftime('1 %Y/%m/%d')."\n";
echo strftime('2 %Y/%m/%d', strtotime('first day'))."\n";
echo strftime('3 %Y/%m/%d', strtotime('last day'))."\n";
echo strftime('4 %Y/%m/%d', strtotime('next month'))."\n";
echo strftime('5 %Y/%m/%d', strtotime('first day next month'))."\n";
echo strftime('6 %Y/%m/%d', strtotime('last day next month'))."\n";
?>
1 2010/02/20
2 2010/02/01
3 2010/02/28
4 2010/03/20
5 2010/03/01
6 2010/03/31

Actually, there is a bug here "first day" and "last day" should be "+1 day" and "-1 day".

This bug has been fixed in SVN.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

Re-opening because of the question marks:

firstdayof = 'first day of'?;
lastdayof = 'last day of'?;

It shouldn't cause an issue though, but it needs fixing.

This may be fodder for another bug report, but strtotime() more specifically 
"returns wrong results" for the last three days of March 2010.

date_default_timezone_set('America/New_York');
echo PHP_VERSION."\n".
  date(DATE_RFC822)."\n".
  date(DATE_RFC822,strtotime('last month'));

5.3.1
Wed, 31 Mar 10 08:21:30 -0400
Wed, 03 Mar 10 08:21:30 -0500

This was also posted as a PHP.net comment: 
http://www.php.net/manual/en/function.strtotime.php#97065

I'm not sure there would be consensus on my expected return values here, but it 
makes more sense then getting March as a result.

Reproduce code:
---------------
date_default_timezone_set('America/New_York');
echo date(DATE_RFC822,strtotime('march 31 2010 -1 month'));
echo date(DATE_RFC822,strtotime('march 30 2010 -1 month'));
echo date(DATE_RFC822,strtotime('march 29 2010 -1 month'));
echo date(DATE_RFC822,strtotime('march 28 2010 -1 month'));


Expected result:
----------------
Sun, 28 Feb 10 00:00:00 -0500
Sun, 28 Feb 10 00:00:00 -0500
Sun, 28 Feb 10 00:00:00 -0500
Sun, 28 Feb 10 00:00:00 -0500


Actual result:
----------------
Wed, 03 Mar 10 00:00:00 -0500
Tue, 02 Mar 10 00:00:00 -0500
Mon, 01 Mar 10 00:00:00 -0500
Sun, 28 Feb 10 00:00:00 -0500

This one is a clearer case of strtotime being bad:

echo date("Y-m-d",strtotime("february", strtotime("march 31 2010")));
2010-03-03

@marques: You're wrong, that's perfectly correct behaviour for last month/previous month. It is also unrelated to this report.

@derick can you comment on what exactly should be the behavior of strtotime('-1 
month') or strtotime('last month')?

today 3/31/10  date('Y-m-d', strtotime('-1 month')) returns '2010-03-03'
               date('Y-m-d', strtotime('+1 month')) returns '2010-05-01'

-1 month seems to offset by 28 days.  +1 month seems to offset by 31 days.  is 
this correct behavior?

@wayne: http://derickrethans.nl/obtaining-the-next-month-in-php.html

Regarding "Re-opening because of the question marks ... It shouldn't cause an issue though, but it needs fixing.", that was fixed today.