What is the difference between MySQL, MySQLi and PDO?

There are (more than) three popular ways to use MySQL from PHP. This outlines some features/differences PHP: Choosing an API:

1. (DEPRECATED) The mysql functions are procedural and use manual escaping.
2. MySQLi is a replacement for the mysql functions, with object-oriented and procedural versions. It has support for prepared statements.
3. PDO (PHP Data Objects) is a general database abstraction layer with support for MySQL among many other databases. It provides prepared statements, and significant flexibility in how data is returned.

I would recommend using PDO with prepared statements. It is a well-designed API and will let you more easily move to another database (including any that supports ODBC) if necessary.

Those are different APIs to access a MySQL backend

• The mysql is the historical API
• The mysqli is a new version of the historical API. It should perform better and have a better set of function. Also, the API is object-oriented.
• PDO_MySQL, is the MySQL for PDO. PDO has been introduced in PHP, and the project aims to make a common API for all the databases access, so in theory you should be able to migrate between RDMS without changing any code (if you don't use specific RDBM function in your queries), also object-oriented.

So it depends on what kind of code you want to produce. If you prefer object-oriented layers or plain functions...

My advice would be

1. PDO
2. MySQLi
3. mysql

Also my feeling, the mysql API would probably being deleted in future releases of PHP.

mysqli is the enhanced version of mysql.

PDO extension defines a lightweight, consistent interface for accessing databases in PHP. Each database driver that implements the PDO interface can expose database-specific features as regular extension functions.