I have done code for scheduling the event in php.
As I schedule it, it goes to user's calender as an invitation.
If I reschedule it, then new invitation goes with same matter with new values.
But I found it is not overwriting the previous one.
How can I do that??

If you ask me for code the its simple mail invitation in php.
While recheduling, i nsend new values to that function.
But tell what should do in code so that when my rescheduled invitation goes in the same calender then it should overwrite previosu one.
Hope am clear with my question.
Need help.

But I found it is not overwriting the previous one.

What exactly is not being overwritten?

simple mail invitation in php.

There is no way to delete mail that is already send.
That has to be done by the receiver.

I mean to say,
when I do reschedule it, My calender shows two invitation
For example: previous one is from 1pm to 2pm
and next is 1pm-3pm

Then it shows me both invitation in my calender,
In zimbra as well as in outlook

Ah, so you want to send a cancellation to Outlook?

yaah , but cancellation should include the over write as I have mentioned above case.