Please help us help you and fill where relevant:
LimeSurvey version: 6.17.8
Own server
Survey theme/template: irrelevant
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My customer want to upgrade his server to Red Hat Linux 10, MySql 8.4.8, NGINX or Apache, highest version.

In the requirements for Limesurver at  www.limesurvey.org/manual/Installation_-_LimeSurvey_CE  
I read:

PHP version requirements:

• LS 7.x from PHP 8.1.29 and up (Tested up to PHP 8.4.x)
• LS 6.x from PHP 7.4.x to 8.3.x

Is it possible to upgrade the new server to PHP 8.5, while still working with LimeSurvey 6.17.9 (which will be upgraded to 7.x when stable enough).

Tammo

 

I think you answered this yourself.

there will no guarantee that this works. I guess there is a reason why the developers mention PHP 7.4.x to 8.3x for LS 6.x.