I am not aware of PHP 5.x, but surely in PHP 7.0 keeping mysql extension creates issues as it doesn't as well comes as part of php.ini file in PHP 7.x
The order of installation doesn't matters, I usually start with Web server, Application server and finally DB server.
For screen going blank, this is absolutely normal and the only solution is deleting cache's - so far I only experienced it on Windows platform with Apache.