I'm familiar with all the changes in PHP from version 4 to 7, and the minor version changes, yet there is no problem in fixing all the code changes, and about choosing between MySQLi and PDO is just regarding what best fits the code you currently have. If it is mostly objective style, it would be probably better to use PDO, if not, it would be better just to translate MySQL driver commands to MySQLi. Either way can be done fast and efficient.