am assuming you want to get a unix timestamp.
just use strtotime('+ 2 days')
EDIT
//insert invoice table
$currentdate = time();
$duedate = strtotime('+2 days');
$makeinvoice = mysql_query("INSERT INTO invoices (userid, hostid, price, ispaid, detecreated, duedate) VALUES ('$userid', '$prodid', '$initprice', '0', '$currentdate','$duedate' )",$conn);
P.S, you should use the mysqli
extention as mysql
is old and would be discontinued soon. Or better still, use pdo