You don't need any special software to write a calendar program. I wrote a calendar program several months ago and I think the only thing that I needed was a leap year function. I found everything else I needed on the internet. I'd start with a pre-made app for this one, it makes things go so much more quickly and smoothly.
I'd try PEAR:
http://pear.php.net
The first I see is located here:
http://pear.php.net/package/Calendar
PHPBuilder:
http://www.phpbuilder.com
Or google:
http://www.google.com/search?q=PHP+calendar
(or calender, a common mispelling)
There are lots of tutorials and free scripts already written and floating around.
HTH!
Regards,
Rich
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The Spicy Peanut Project
http://www.spicypeanut.net
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