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')