Exemplo conexão com PDO - CloudSql

<?php

echo "Conectando...";
$host = "mysql:unix_socket=/cloudsql/ID_INSTANCIA;dbname=NOME_BANCO";
$user = "USUARIO";
$password ="SENHA";

try{
  // create a PDO connection with the configuration data
  $conn = new PDO($host, $user, $password);

  // display a message if connected to database successfully
  if($conn){
    echo "Connected database successfully!";
  }

 } catch (PDOException $e) {
  // report error message
  echo $e->getMessage();
 }
?>