Query String

Hi, I have developen one web application in php where in i have to pass many values like user id, order id, session id etc in query string for proper functioning. The site is working perfectly. The problem is that when i change any one of the query string value, the respectie page gets loaded. logically speaking this will happen if we change the query string value. But i want to know how to avoid this.
For example:
My website has URL similar to [url]www.mysite.com/index.php?Id=1&p=orderlist[/url]

If i change the value of Id to 2, the respective contnt will get loaded. This shouldnt happen in my case. So please if some one now this, help me.

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