PHP:无法找到驱动程序(PDO, MySql)

<?php

error_reporting( E_ALL & ~E_DEPRECATED & ~E_NOTICE );
ob_start();
session_start();

define('DB_DRIVER', 'mysql');
define('DB_SERVER', 'localhost');
define('DB_SERVER_USERNAME', 'root');
define('DB_SERVER_PASSWORD', '');
define('DB_DATABASE', 'db_test');

define('PROJECT_NAME', 'Testing create');
$dboptions = array(
        PDO::ATTR_PERSISTENT => FALSE,
        PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
        PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAME utf8',
    );

try{
    $DB = new PDO(DB_SERVER. ':host='.DB_SERVER. ';dbname='.DB_DATABASE,DB_SERVER_USERNAME, DB_SERVER_PASSWORD , $dboptions);
}catch (Exception $ex) {
    echo $ex->getMessage();
    die;
}

require_once 'functions.php';

if($_SESSION["errorType"] != "" && $_SESSION["errorMsg"] != "" ){
    $ERROR_TYPE = $_SESSION["errorType"];
    $ERROR_MSG = $_SESSION["errorMsg"];
    $_SESSION["errorType"] = "";
    $_SESSION["errorMsg"] = "";
}

?>
<?php

error_reporting( E_ALL & ~E_DEPRECATED &