Calculate "working" days difference.

I'm wondering if it's possible to calculate the difference in days between to dates and exclude saturdays and sundays? Currently I just use:

PHP Code:

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    $diff = strtotime($date2) - strtotime($date1);
    $et = $diff/(3600*24); 


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but that gives me the difference in calendar days. How do I tell it not to count the weekends? Holidays would be good to exclude, but that's far too complicated as it would change, so I'm not doing that. But getting the working days passed would be of great value.