In magento we can set session ,get session and unset session custom session variable value in magento.

<?php

// For Set custom session in Magento 
$setSessionVariable =  Mage::getSingleton('core/session')->setYoursessionname('you session value');

// For Get custom session in Magento 
echo $getSessionVariable =  Mage::getSingleton('core/session')->getYoursessionname();

// For Unset custom session value in Magento 
  Mage::getSingleton('core/session')->unsYoursessionname();

 ?>

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