The easiest solution I can think of off the top of my head is to check the weekday and increment if the days are weekends.
ie.
Code:
//pseudocode, I didn't test this, its just concept
function addWorkDays(startDate, numDays) {
$numDaysPadding = 0;
for ($i = 0; $i < numDays; $i++) {
if ( $startDate + ($i +1) == "Saturday" ||
$startDate + ($i +1) == "Sunday") {
$numDaysPadding++;
}
}
$numDays += $numDaysPadding;
return $startDate + $numDays;
}
Essentially, you increment the number of days your adding if you encounter a weekend day. Zend_Date can return the actual day of the week via ->getWeekday(). This returns an int which is mapped to the day of the week:
0 => Sunday
1=> Monday
2 => Tuesday
3 => Wednesday
....
6 => Saturday
hope that helps...