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If it is an URL parameter you can use

$pageId = Mage::app()->getRequest()->getParam('pageid', 0)

For category pages:

$pageId = Mage::registry('current_category')->getId()

You can also modify your single category from your_layout.xml Layout handles for categoris look like:

<CATEGORY_{yourId}>
    <reference ...

For CMS pages:

$pageId = Mage::getSingleton('cms/page')->getId() // use getIdentifie‌​r()