Extending PDO class: why does the method return 0 instead of error message?

PHP
1

Hi,

I am trying to extend the pdo class, it seems that I have managed to extend it,

class database_extended extends PDO
    {
    	
    	#make a connection
    	public function __construct($dsn,$username,$password)
    	{
    		try 
    		{ 
    			parent::__construct($dsn,$username,$password);
    			//$this->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    		}
    		catch (PDOException $e) 
    		{
    			# call the get_error function
    			self::get_error($e);
    		}
    	}
    	
    	#get the number of rows in a result
    	public function num_rows($query)
    	{
    		try 
    		{
    			# create a prepared statement
    			$stmt = parent::prepare($query);
    			
    			# execute query 
    			$stmt->execute();
    			
    			# return the result
    			return $stmt->rowCount();
    		} 
    		catch (PDOException $e) 
    		{
    			# call the get_error function
    			self::get_error($e);
    		}
    	}
    	
    	# display error
    	public function get_error($e) 
    	{
    		$this->connection = null;
    		die($e->getMessage());
    	}
    	
    	# closes the database connection when object is destroyed.
    	public function __destruct()
    	{
           
        }
    }

But it seems not quite right - I tested the num_rows method with a intended mistake in the query, so this method can return an error message,

# the host used to access DB
    define('DB_HOST', 'localhost');
    
    # the username used to access DB
    define('DB_USER', 'root');
    
    # the password for the username
    define('DB_PASS', 'xx');
    
    # the name of your databse 
    define('DB_NAME', 'xx_2011');
    
    # the data source name
    define('DSN', 'mysql:dbname='.DB_NAME.';host='.DB_HOST);
    	
    include 'class_database.php';
    
    $connection = new database_extended(DSN,DB_USER,DB_PASS);
    
    $sql = "
    	SELECT *
    	FROM table_not_exist
    	ORDER BY cnt_id DESC
    	";
    	
    echo $connection->num_rows($sql);

It should returns,

SQLSTATE[42S02]: Base table or view not found: 1146 Table 'xx_2011.table_not_exist' doesn't exist

But it returns a 0 instead! Why?? How can I fix it?

Thanks.