老是出现boolean false

<?php

try {

    $pdo = new PDO('mysql:host = localhost;dbname = test', 'root', '');

    //exec():执行一条sql语句返回受影响的记录的条数，去过没有受影响的记录，他返回0

    //exec对select不起作用

    $sql = <<<EOF

    create table if not exists user(

    id int unsigned auto_increment key,

    username varchar(20) not null unique,

    password char(32) not null,

    email varchar(30) not null

    );

EOF;

    $res = $pdo ->exec($sql);

    var_dump($res);

} catch (PDOException $e) {

    $e ->getMessage();

}