Indeed, there is much crappy PHP appliaction code out there in the wild. Anyways … when you have to care about webservers you may have systems which are relying on PHP. Upgrading this systems to the next major PHP version may break projects hosted on those systems, cause there is deprecated code in them which is not supported by the new PHP version.
I just hacked together a smallish shell script to spot problematic functions and ini directives of the working directory. Enhancements and missing code fragments are welcome.