What is the difference between PDO and MySQLi?

Both are PHP extensions for database interaction, but they differ in scope and philosophy. PDO (PHP Data Objects) is a database abstraction layer supporting 12+ database drivers — switching from MySQL to PostgreSQL requires only changing the DSN connection string. It uses a unified API regardless of the underlying database. MySQLi (MySQL Improved) is MySQL-specific but provides both an object-oriented and a procedural interface. MySQLi supports all MySQL-specific features (async queries, multi-statements) that PDO might not expose. Both support prepared statements and transactions. The recommendation is always use PDO for new projects — the database portability it provides is invaluable, and its API is cleaner for building abstractions on top of. Use MySQLi only if you need MySQL-specific features PDO does not expose.