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Before 5.4 eZ Publish uses default Framework Bundle configuration, witch which on Symfony 2.3 implies Symfony's NativeFileSessionHandler[3] (session.handler.native_file service). This handler forces PHP's builtin "files" session save handler, and specifically configures it to use session.save_path set to ezpublish/sessions by default (ezpublish/cache/<env>/sessions before 5.3).
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