Back button on web browser

Could anyone help me with this:
I have seen some web program can control "back" button on the browser, so when the "Back" button on the BROWSER is clicked, cookie is sent out to verify the valid users before they can even see the previous page. But I couldn't find out how this control is done by using either HTML or PLSQL packages? I would really like to use this feature in my program.

Thank you for any help.

Zhuang

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