Hey OP i’m in similar situation. I know basics of PHP and started learning PHP OOP while ago. I can’t recommend highly enough Matt Zandstras book “PHP objects, Patterns, and Practice“. If one does not learn OOP with this book all hope is gone.
What comes to accessing databases i would say use PDO class. Yes, its bit slower than native MySQLI, but once you fully understand PDO you can use it with all other databases as well. MySQL is dying (not anytime soon, but sooner or later) so it wiser to put your bets on horse that will win in longer run.
EDIT: Just noticed OP date. scratching head