the number of concurrent database connections is only limited by your database server/hosting, and is typically several 100. your script could (but shouldn't) make a dozen connections using both mysqli and PDO at the same time. the PDO extension is better designed, simpler, and more consistent than the mysqli extension. as an added benefit, works with 12 different database types, so you are not learning a new set of php statements for each database type. if you had some problem using t