$pdo->execute Выдает ошибку

<?php
    function __autoload($classname)
    {
    require_once(__DIR__."\\$classname.php");
    }
    $dbh =  Lib\Connect::getInstance();
    $tab=$dbh->prepare('SELECT username, email, date, message FROM messages ORDER BY :column :course LIMIT :number, :count');
    $tab->execute(["column"=>"username","course"=>"DESC","number"=>"0","count"=>"5"]);
    while($val=$tab->fetch()) print_r($val);

<?php
namespace View;
 
class Content
{
    public function __construct($ob, $col='username',$num='0',$coun='20', $cour='DESC')
    {
        $this->dbh = $ob;
        $this->course = $cour;
        $this->column = $col;
        $this->number = $num;
        $this->count = $coun;
        $this->tabheader();
        $this->content($ob);
    }
    public function getInstance()
    {
    return '<div class="content"><table align="center" >'.$this->headtab.$this->messages.'</table></div>';
    }
    private $count;
    private $course;
    private $column;
    private $headtab;
    private $messages;
    private $number;
    private function tabheader()
    {
    $cls = ['username'=>'"table"','email'=>'"table"','date'=>'"table"','usernameroute'=>'"uroute"','emailroute'=>'"uroute"','dateroute'=>'"uroute"'];
    $cls[$this->column]= 'table1';
    $cls[$this->column.'route']=$this->course.'route';
    $this->headtab = "<tr>
    <td><div class=$cls[username]><div class=$cls[userroute]></div>Пользователь</div></td>
    <td><div class=$cls[email]><div class=$cls[emlroute]></div>E-mail</div></td>
    <td><div class=$cls[date]><div class=$cls[dateroute]></div>Дата</div></td>
    <td><div class=\"table\">Сообщение</div></td>
    </tr>";
    }
    private function content($dbh)
    {
        $res = $dbh->query('SHOW TABLES LIKE "messages"');
        if($res->rowCount()=== 0)
        {
            $query='CREATE TABLE messages (
            date DATETIME NOT NULL,
            username TEXT NOT NULL,
            email TEXT NOT NULL,
            homepage TEXT,
            message MEDIUMTEXT NOT NULL,
            ip TINYTEXT NOT NULL,
            client TEXT,
            appname TEXT
            )';
            $tabl=$dbh->exec($query);
        }
        $query="SELECT username, email, date, message FROM messages ORDER BY $this->column $this->course LIMIT $this->number, $this->count";
        $ins=$dbh->query($query);
           foreach($ins->fetchAll() as $val)
        {
            $this->messages.= '<tr>
            <td><div class="table">'.$val['username'].'</div></td>
            <td><div class="table">'.$val['email'].'</div></td>
            <td><div class="table">'.$val['date'].'</div></td>
            <td><div class="table">'.$val['message'].'</div></td>
            </tr>' ;
         }
    }
 
}