I did an upgrade on my Desktop at home from Jesse to Stretch a while back, and in the move went from php5 to php7.1..
Well phpMyAdmin kept giving me an error saying that MySQLi was not installed or enabled.
Turns out MySQLi for php7.1 was install, but not php7.1.. Not really sure how it managed that in the upgrade.
Did a apt-get remove for php5
apt-get remove php5 libapache2-mod-php5 php5 php5-cli php5-common php5-curl php5-gd php5-json php5-mcrypt php5-mysql
Then did a install of php7.1 (phpMyAdmin also complained it need mbstring)
apt-get install php7.1 php7.1-mbstring
Then restarted apache