使用OOP Php获取所有结果

I've been following a tutorial about oop php on youtube. He made a method to fetch only the first result of a query. I tried to grab all results, but I get a warning message : Trying to get property of non-object

Also when I var_dump(); my object holding the result (which I assumed), it returns null.

What am I doing wrong?

This is the full code:

DB class:

class DB {

     private $_results;

     public function query($sql, $params = array()) {

        $this->_error = false;

        if ($this->_query = $this->_pdo->prepare($sql)) {

            $x = 1;

            if (count($params)) {

                foreach ($params as $param) {

                    $this->_query->bindValue($x, $param);

                    $x++;

                }

            }

            if ($this->_query->execute()) {

                $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
                $this->_count = $this->_query->rowCount();

            } else {

                $this->_error = true;

            }

        }

        return $this;

    }

     // First result only:

     public function first() {

          return $this->results()[0];

     }

     // All results as I thought:

     public function first() {

          return $this->results();

     }

     public function results() {

          return $this->_results;

     }

}

Project Class:

class Project {

    private $_db,
            $_data;

    public function __construct() {

        $this->_db = DB::getInstance();

    }

    public function find($user = null) {

        if ($user) {

            $data = $this->_db->get('user_project', array('uid', '=', $user));

            if ($data->count()) {

                $this->_data = $data->all();

                return $this->data()->id;

            }

        }

    }

    public function data() {

        return $this->_data;

    }

}

I tried to access it by doing this:

$project = new Project();

var_dump($project->find($user->data()->id)); // $user->data()->id is just the id of a user

Thanks for pointing me the right direction, I've figured it out.

 $this->_data = $data->results();

 return $this->_data;

Then I had to loop through it. Solved.