Two ways I know of - but you need your invoice date in a Unix timestamp first. You can use strtotime instead of mktime.
// say invoice date is Feb 24 2000
$idate = mktime (0,0,0,2,24,2000);
echo "Invoice ",date ("M-d-Y", $idate),"<br>";
// Method 1: add 11 days, in seconds, to get duedate
$ddate = $idate + (11 * 86400);
echo "Due (1) ",date ("M-d-Y", $ddate),"<br>";
// Method 2: add 11 days and use mktime to get duedate
$idate = mktime (0,0,0,2,24+11,2000);
echo "Due (2) ",date ("M-d-Y", $ddate),"<br>";
Note that mktime happily caters for out of range days when adding or subtracting.