I would like to answer the question in a simple way is here First and the most important difference is mysqli supports MySQL only while PDO, PHP Data Object, is a common class for certain databases including MySQL and Postgres.
You should note that PDO is maintained by PHP community, unlike connectors except for MySQL. Also, mysqli is going to be deprecated in future versions of PHP. And if you are using database any other than MySQL then you don't have any option apart from PDO, considering native PHP modules.