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// for MySQL
$conn = new PDO("mysql:host=$host;dbname=$db", $user, $pass);

// for SQLite
$conn = new PDO("sqlite:$db");

// for postgreSQL
$conn = new PDO("pgsql:host=$host dbname=$db", $user, $pass);

Look following code:

<?php
// configuration
$dbtype		= "sqlite";
$dbhost 	= "localhost";
$dbname		= "test";
$dbuser		= "root";
$dbpass		= "admin";
$dbpath		= "c:/test.db";

// switching
switch($dbtype){
  case "mysql":
    $dbconn = "mysql:host=$dbhost;dbname=$dbname";
    break;
  
  case "sqlite":
    $dbconn = "sqlite:$dbpath";
	break;
  
  case "postgresql":
    $dbconn = "pgsql:host=$host dbname=$db";
	break;
}

// database connection
$conn = new PDO($dbconn,$user,$pass);

?>

This is sample mode to write connection. It is more portable, because when we change database, we just change at configuration part (may be only change line 3).