在管理员菜单上调用字符串上的成员函数prepare（）

Question

I created a custom admin menu:

function new_menu() {
    add_menu_page(
        'New Menu',
        'New Menu',
        'administrator',   
        'new_menu',
        'new_menu_content'
    );
}

add_action('admin_menu', 'new_menu');

Inside that new menu, I get some data from a Database:

function fishing_data_content($conn){
    require_once(get_template_directory() . '/connect.php');
    require_once(get_stylesheet_directory() . '/data.php');
}

The connect.php connects to the DB:

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "data";
try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, 
$password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);    
}catch(PDOException $e){
    echo "Error: " . $e->getMessage();
}

And the data.php fetches some data from that DB:

$stmt = $conn->prepare('SELECT * FROM posts');
    $stmt->execute();
    $values = $stmt->fetchAll();
    foreach($values as $val){
        echo $val . '<br>';
    }
}

Also I tried to require the connection file inside the data.php:

require_once(dirname(__FILE__) . '/connect.php');

But still having the same error:

Fatal error: Call to a member function prepare() on string in /home/public_html/wordpress/wp-content/themes/theme/data.php on line 10