might be daylight saving time related?

it's always better to use mktime(12,0,0,...
instead of mktime(0,0,0,... 
when you care about date only but not time ...

There is no Novemeber 31st, this is why it is returning December; it skips to the equivalent day which would be the 1st of Dec. Same with Septemeber.

This is desired behavior.

-Chris

I'm disagree with cnewbill.

For example in 30 Apr, next was 01 May, and in 30 Jun next was 01 Jul.

Why in october doesn't it work?

Again same result and same reason.  What does April 30th and June 30th have to do with your problem??

Last I checked the calendar works like this

So you're telling me that 9/31 isn't the same calendar day as October 1st and Novemember 31st isn't the same calendar day as Dec 1st??

Sept   Oct          Nov          Dec
30     1 2 3 ... 31 1 2 3 ... 30 1 2 3

<?php

print "May\n";
print "Before: ".date("m", mktime(0,0,0,(5-1),31,2001))."\n";
print "After: ".date("m", mktime(0,0,0,(5+1),31,2001)). "\n";

print "October\n";
print "Before: ".date("m", mktime(0,0,0,(10-1),31,2001))."\n";
print "After: ".date("m", mktime(0,0,0,(10+1),31,2001)). "\n";

?>

[cnewbill@storm cnewbill]$ php -q t.php
May
Before: 05
After: 07
October
Before: 10
After: 12

-Chris