Object of class PDOStatement could not be converted to string in how to fix?

Question

PHP

Good afternoon! I'm trying to change the database connection from mysqli to PDO, at this stage I get an error. Help me understand what the problem is.
The error is gone, but the requests are not being fulfilled.
PS Thanks in advance.

class DB {

    protected $connection;

    public function __construct($host, $user, $password, $db_name){
        //$this->connection = new mysqli($host, $user, $password, $db_name);
        $this->connection = new PDO('mysql:dbname=news_db;host=localhost', $user, $password);
        /*if( mysqli_connect_error() ){
            throw new Exception('Could not connect to DB');
        }*/
        try {
            $dbh = $this->connection;
        } catch (PDOException $e) {
            echo 'Подключение не удалось: ' . $e->getMessage();
        }
    }

    public function query($sql){
        if ( !$this->connection ){
            return false;
        }

        $result = $this->connection->query($sql);

        /*if ( mysqli_error($this->connection) ){
            throw new Exception(mysqli_error($this->connection));
        }*/

        if ( is_bool($result) ){
            return $result;
        }

        $data = [];
        /*while( $row = mysqli_fetch_assoc($result) ){
            $data[] = $row;
        }*/
        while ($row = $result->fetchAll(PDO::FETCH_ASSOC)) {
            $data[] = $row;
        }
        return $data;
    }

    public function escape($str) {
        //return mysqli_escape_string($this->connection, $str);
        return $this->connection->quote($str);
    }
}

Request

public function getByAlias($alias){
        $alias = $this->db->escape($alias);
        $sql = "select * from pages where alias = '{$alias}' limit 1";  // ошибка здесь или ранее
        $result = $this->db->query($sql);
        return isset($result[0]) ? $result[0] : null;
    }

Model class, may need

class Model
{

    protected $db;

    public function __construct()
    {
        $this->db = App::$db;
    }
}

Answer 1

Well, you probably not only received the text of the error, but also an indication of where it happened.

...
$result = $this->db->query($sql);
return isset($result[0]) ? $result[0] : null;
...

Specifically, the error "Object of class PDOStatement could not be converted to string" occurs here.
In $result you will have a PDOStatement object after executing query(). To get a result from it, you need to execute
Or like this:

...
$result = $this->db->query($sql)->fetchAll();
return isset($result[0]) ? $result[0] : null;
...

Answer 2

Can you tell me where it comes from.