How to display errors or where is the bug?

Question

PHP

<?php
require_once('core/db.php');
require_once('core/Data_object.php');
require_once('db_rating.php');

Class ratings
{    
    protected $widget_id;  
    protected $data;
    private   $DB_Rating = new DB_Rating();
  
  function __construct($wid) {
    $this->data = $this->DB_Rating->get($wid);
    	$this->widget_id = $wid;
  }

  public function get_ratings() { 
    $Data_object = new Data_object();

    $Data_object->widget_id = $this->widget_id;
    $Data_object->number_votes = 0;
    $Data_object->total_points = 0;
    $Data_object->dec_avg	   = 0;
    $Data_object->whole_avg	   = 0;
    	
    	foreach($this->data as $vote)
    	{
    		$Data_object->number_votes += 1;
    		$Data_object->total_points += $vote->vote;
    	}
    	$Data_object->dec_avg = $Data_object->total_points / $Data_object->number_votes;
    	$Data_object->whole_avg = round($Data_object->dec_avg);

    	echo json_encode($Data_object->get_filds_in_assoc_array());
  }

  public function vote() {
      $Data_object = new Data_object();

      preg_match('/star_([1-5]{1})/', $_POST['clicked_on'], $match);  
      $vote = (integer) $match[1];
    $ID = $this->widget_id;  
    
    session_start();
    $Data_object->set_fields(
      'vote', 	  $vote,
      'company_id', $ID,
      'user_id',	  (integer) $_SESSION['user_id']
    );
    $this->DB_Rating->set_data($Data_object);
    $this->DB_Rating->set();

    $this->get_ratings();
  }
}

<?php

Class DB_Rating extends DB
{
  protected $select_query = 'SELECT * FROM rating INNER JOIN users USING(user_id) WHERE company_id = ?';

  protected $insert_query = 'INSERT INTO rating';
  protected $set_vote  		= 'vote = ?';
  protected $set_company_id 	= 'company_id = ?';
  protected $set_user_id 		= 'user_id = ?';

  protected $check_query = 'SELECT * FROM rating WHERE company_id = ? AND user_id = ?';


  public function __construct($obj = '')
  {
    $this->pdo = new PDO($this->dsn, 'root', '', $this->opt);
        $this->Data_object = $obj;
  }

  public function get($company_id)
  {
    if (isset($company_id))
        {
            $Data_object = new Data_object();
    
      $stmt = $this->pdo->prepare($this->select_query);
      $stmt->execute(array($company_id));
      $j = 0;
      foreach($stmt as $request_vote)
      {
        $Vote = new Data_object();
        $Vote->set_fields
        (
          'user', $request_vote['login'],
          'vote', $request_vote['vote']
        );
        $name = 'vote_'.$j;
        $Data_object->$name = $Vote;
        $j++;
      }
      return $Data_object;
        }
  }

  public function set()
  {
    if (isset($this->Data_object) || is_object($this->Data_object))
        {
            $names_of_fields = $this->Data_object->get_names();
            for ($i = 0; $i < count($names_of_fields); $i++)
            {
                $this->filter('INSERT INTO rating', 'insert', $names_of_fields[$i], 'set', ' SET ', ', ');
            }
            $arr = $this->Data_object->get_filds_in_array();
            if (
            	isset($arr) && 
            	$this->insert_query != 'INSERT INTO rating' && 
            	$this->isclone( $check_query, $this->Data_object->company_id )
            ) {
                $stmt = $this->pdo->prepare($this->insert_query);
                $stmt->execute($arr);
            }
        }
        else {
            throw new Exception("It must be object.");
        }
  }

  private function isclone($query, $fild_name)
  {
    session_start();
    $name = $this->Data_object->get_fields
    (
      'company_id', $fild_name,
      'user_id', $_SESSION['user_id']
    );

        if (isset($name))
        {
            $check = $this->pdo->prepare($query);
            $check->execute(array($name));
            if (!$row = $check->fetch(PDO::FETCH_LAZY))
            {
                return true;
            }
            else {
                echo '$err_text_1';
                return false;
            }
        }
        else {
            echo '$err_text_2';
            return false;
        }
  }
  }

  public function set_data($Data_object)
    {
    	$this->Data_object = $Data_object;
    }
}

Data_object - a class for collecting, formatting, transferring and convenient presentation of data.