There is no need to call session_start anywhere in your code: eZPublish always starts a session for every visitor of the site, even anonymous ones. If you want to access (or set) session variables from templates, you can create custom template operators.
Principal Consultant International Business
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Alternatively if this is for logged in users you can use ez preference witch is supported by the view cache system, and therefor will save you some headache. You set them with user/preferences/<key>/<value> and it will redirect back to referrer, and you control which one of them affect view cache with site.ini[ContentSettings]CachedViewPreferences.
