What is the difference between MySQL, MySQLi and PDO?

MySQL The mysql functions are procedural and use manual escaping.

// mysql

$mysql = mysql_connect('localhost','username','password','database');

MySQLi is a replacement for the mysql functions, with object-oriented and procedural versions. It has support for prepared statements.

// mysqli, procedural way

$mysqli = mysqli_connect('localhost','username','password','database');

// mysqli, object oriented way

$mysqli = new mysqli('localhost','username','password','database');

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.

// PDO

$pdo = new PDO("mysql:host=localhost;dbname=database", 'username', 'password');