I wanted to provide a fix that should address issues with PHP version 8.3 by fixing core/URI.php: Declaring the $config property at the top of the CI_URI class:

phpCopyclass CI_URI {
    protected $config;
    // Rest of the existing class code

As well as fixing the database/DB_driver.php by adding the $failover property to the CI_DB_driver class:

phpCopyclass CI_DB_driver {
    protected $failover;
    // Rest of the existing class code

I am not getting any errors from Invoiceplane after implementing these fixes, but I wanted to make sure that these fixes do not break anything else.

I know that newer versions of php may not be supported, but for folks who don't compartmentalize every single web app they run and share the same version of php, this could be a fix.