Derick Rethans (master of all things date and time in PHP) has a new post to his blog showing how to correctly get the "next month" since the date string parser doesn't understand how to use that string directly.

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.

He describes what happens internally, how PHP bumps up the month and counts the number of days, and how you can use some of newer features in the PHP 5.3 release to get the first day of the next month. Specifically, this means using the "first day of next month" string for the modify() call on the DateTime object. If you're not on PHP 5.3 yet, Derick also includes a method in the comments to do it a slightly different way.