$host     = "localhost";//Ip of database, in this case my host machine    
$user     = "root";	//Username to use
$pass     = "qwerty";//Password for that user
$dbname   = "DB";//Name of the database

try {
    $connection = new PDO("mysql:host=$host;dbname=$dbname", $user, $pass);
    $connection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

}catch(PDOException $e)
{
  echo $e->getMessage();                         
}

<?php
$pdo = new PDO('mysql:host=localhost;dbname=databasename', 'username', 'password');
?>

$hostName = "localhost";
$dbName = "test";
$userName = "test";
$password = "test1";
try {
    $pdo = new PDO("mysql:host=$hostName;dbname=$dbName",$userName,$password);
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    echo "Connected successfully"; 
    }
    catch(PDOException $e)
    {
     echo "Connection failed: " . $e->getMessage();
    }

<?php 
  $db_src    = "mysql:host=localhost;dbname=databasename";
  $db_user   = "root";
  $db_pass   = "";
  try{
    $connect = new PDO($db_src, $db_user, $db_pass);
    $connect->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
  }
  catch(PDOEXCEPTION $err){
    echo "Failed To Connect. Error " . $err->getMessage();
  }
/* Variable $connect will be used to execute DB Statements*/

$db = new PDO("sqlsrv:Server=YouAddress;Database=YourDatabase", "Username", "Password");

$connect = new PDO("mysql:host=$host;dbname=$db", $user, $pass, array(PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));