// Create a pdo instance
$host = 'localhost';
$database_name = 'databasename';
$username='username by default it is root';
$password='the password associated with the current user';

try{
  // check if there is exception
  $pdo = new PDO("mysql:host=$host;dbname=$database_name","$username","$password");

  // now we've connected successfully to $database_name database
  // and we can do whatever we want with it.
}
catch(PDOException $e){
  // show a message explaining what is goining wrong.
  echo $e->getMessage();
}