How to convert MySQLi functions into PDO functions

I’m trying to re-write my app’s backend to be PDO based instead of MySQLi based. I’m switching from an SQL database into something a little more home-grown and intricate so I need to use PDO instead. I’m not used to PDO and learnt PHP with MySQLi so excuse me if these questions seem a little ignorant.

I’m trying to convert some functions as follows:
$url_address = $first_name . "." $last_name; & $userid = create_userid(); from working MySQLi functions into working PDO functions.