I use PHP language in my application and use these two drivers to connect to MySQL - mysqli and pdo. I know that PHP is licensed under PHP License v3.01, so I can freely use it, but at the same time, since I connect to MySQL which is licensed under stricter GNU GPL, I must know how MySQL drivers are licensed. I could not find such information on the Internet and I do not know whether mysqli and pdo are licensed under PHP License v3.01 or under GNU GPL.
Both the mysqli and the PDO database drivers are distributed as part of the PHP language distribution and are compiled into the PHP executable.
As there is no indication that the license of PHP changes when either of these drivers is enabled, they cannot be distributed under a copyleft license like the GPL. This in turn means that using the mysqli or PDO drivers in your PHP application cannot force you to use a copyleft license for your own code.