How to use session in opencart

as mentioned, how to use session
PS:
in opencart to get a development manual of version 2.0.1 of opencart, please leave a message.
Thank you!

Php
Feb.26,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1eaab38-1ffe.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1eaab38-1ffe.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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