PHP error after AvantSearch 4.0 install

[jefke-glider]

I installed the AvantSearch module from github, the latest edition (4.0). My Omeka Classic is 3.1.1 (I know, this needs an update)and my PHP is version 7.4.3. When I edit an item, I get this error :

PHP Parse error: syntax error, unexpected '|', expecting variable (T_VARIABLE) in /var/www/omeka-3.1.1/plugins/AvantSearch/models/SearchPdf.php on line 168,...
I checked the code, and there is a union operator, which requires PHP 8. I patched it locally, ( removed the |string part), but mayb there is a better way to make it php 7 compatible again.