Run bin/generate-from-disabled.php to generate a configuration based on the disable_functions & disable_classes PHP options. The configuration will be dumped to STDOUT in NEON format, you can save it to a file and include it in your PHPStan configuration.
The file needs to be pre-generated because different environments often have different PHP configurations and if disabled functions & classes would be disallowed dynamically, the configuration would vary when executed for example in dev & CI environments.