Migrate website on php 8.1

There is a website he on 7.2 https://skidka-plus.com/

Vps is deployed in the cloud and a test version of the http://213.189.220.147:8888/ is deployed on it
and there are errors http://213.189.220.147:8888/ru/service/

Task
1 Check the site for errors in the PCP i.e. using the IDE or something else to detect errors
2 Discuss
3 And fix them


Work is done locally in the cloud on a server connection via SFTP and SSH
If you have Linux OS you can mount a section of the site yourself and work as a local
Term 1-2 days

PS:The new version is more strict to the code and some points stop working because it was originally written on version 5.6