I run Debian bullseye, just updated to php8.1  as per

https://computingforgeeks.com/how-to-install-php-on-debian-linux-2/

Changed my config to reflect the new socket 

fastcgi_pass unix:/var/run/php/php8.1-fpm.sock;

Now get an error:

Error: Call to undefined function create_function() (0)
#0 /var/www/html/conf_global.php(370): IPB_Firewall::_calculateChecksums()
#1 /var/www/html/conf_global.php(342): IPB_Firewall::_doSecurityChecks()
#2 /var/www/html/conf_global.php(574): IPB_Firewall::run()
#3 /var/www/html/system/Dispatcher/Dispatcher.php(69): require('...')
#4 /var/www/html/index.php(13): IPS\_Dispatcher::i()
#5 {main}

for the time being I have dropped back to PHP7.4

What is my fix?

Please go to your conf_global.php file and remove anything refrencing IPB_Firewall (make a backup of your conf_global.php if you are unsure what is refrencing this). This is not our product and means that at some juncture while running version 3, your hosting was breached and this was added.

Wonderful .. thank you very much 

Yeah my forum is very old 😛 .. probably has been broken into in the past

You're very welcome. Glad we could be of assistance.