This is more complex, is not like u think it ...
U need to check in witch month u are lets say u are in day 30 of month may and u need to show witch day will be after 3 days?
Well after 3 days will be day 2, month June.
See my point? :p
And for month february u need to check in witch year u are ...
And here is an ex. that will help u thinking this over ...
echo date('d M Y',mktime(0,0,0,5,30,2006)); //30 May 2006
echo date('d M Y',mktime(0,0,0,5,30+3,2006)); //2 June 2006