As noted in several blogs, strtotime() solves the "+1 month" ("next month") issue on days that do not exist in the subsequent month differently than other implementations like for example MySQL.
$dt = date("Y-m-d");
echo date( "Y-m-d", strtotime( "$dt +1 day" ) ); // PHP: 2009-03-04
echo date( "Y-m-d", strtotime( "2009-01-31 +2 month" ) ); // PHP: 2009-03-31