You can easily check compatibility by using php7 compatibility when upgrading PHP. When using php7compatibility, some preparation is required for environment setup, but if it is a docker environment, the already created docker image is released, so you can build the environment and check it quite easily. ..
For example, the following is a command example for checking compatibility with PHP 7.1.
You can exclude subordinates from vender from the check with --ignore = vendor.
docker run --rm -v $(pwd):/app vfac/php7compatibility 7.1 --extensions=php --ignore=vendor .