/**
 * 
 * @param String $data 'yyyy-mm-dd' the base date
 * @param Int $nmonth number of months to add
 * @param String $format The output format
 *
 * @return String
 **/
function incrementsMonth($data, $nmonth = 1 , $format = 'Y-m-d')
{	
	$timeStampAddMonth = strtotime( " + $nmonth month " , strtotime( $data ) );
	$vetorData = explode('-',$data);
	$timeStampNovoMes =  strtotime( date('Y-m-t', strtotime( " + $nmonth month " ,   mktime( 0,0,0,$vetorData[1],1,$vetorData[0] ))));
	return ($timeStampAddMonth > $timeStampNovoMes) ? date($format, $timeStampNovoMes) : date($format, $timeStampAddMonth) ;
}

Use this function if you need to get the same day in the next month (or how many you need).
Using defaults functions of php you got this:
echo date('Y-m-d', strtotime( " + 1 month " ,  strtotime('2010-01-31')));  (Output: '2010-03-03')

and if you use the function 'incrementsMonth', you got :
echo incrementsMonth('2010-01-31');  (Output: '2010-02-28')

Increment Month

php, date

PHP