<?php echo S2MEMBER_PAYPAL_PDT_IDENTITY_TOKEN; ?>
This may output something like: jjweu7234jKDdsjSLKSEDFoe38345j3232llii3450823kk2o0jgg
(or whatever you have configured as your PayPal® Identity Token)

---- s2member Shortcode Equivalent ----

[s2Get constant="S2MEMBER_PAYPAL_PDT_IDENTITY_TOKEN" /]