使用PHP include连接创建MySQL表

我试图用php建立一个数据库。
下面是我正在做的:

db_connect.php

<?php
    $db_connect = mysqli_connect("localhost", "mydb", "", "");
    if (mysqli_connect_errno()) {
        echo mysqli_connect_error();
        exit();
    } else {
        echo "Successful database connection, happy coding!!!<br/><br/>";
    }
?>

db_create_table.php

<?php
include_once("db_connect.php");
$tbl_users = "CREATE TABLE IF NOT EXISTS users (
            id INT(11) NOT NULL AUTO_INCREMENT,
            username VARCHAR(16) NOT NULL,
            email VARCHAR(255) NOT NULL,
            password VARCHAR(32) NOT NULL,
            userlevel ENUM('u','m','a') NOT NULL DEFAULT 'u',
            avatar VARCHAR(255) NULL,
            ip VARCHAR(40) NOT NULL,
            signup DATETIME NOT NULL,
            lastlogin DATETIME NOT NULL,
            activated ENUM('0','1') NOT NULL DEFAULT '0',
            PRIMARY KEY (id),
            UNIQUE KEY username (username,email)
            )";
            $query = mysqli_query($db_connect, $tbl_users);
if ($query === TRUE) {
    echo "Table 'users' is created!<br/><br/>"; 
} else {
    echo "Table 'users' is not created!<br/><br/>"; 
}
?>

结果如下:

数据库连接成功，编码愉快!

表'users'未创建!

有人能帮我一下吗，我不知道我的错误在哪里:(