facebook page_id in a tab

hi i am trying to g开发者_如何转开发et facebook page_id on a page in a tab where my application is added but i am getting error.

echo $_REQUEST["fb_page_id"];

Message: Undefined index: fb_page_id

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If you would var dump the $_REQUEST array you would see all the params available for you to use. What you want it is called fb_sig_page_id.