PHP PDO fetchOject的问题

It seems to be a bug or problem when I use PHP PDO fetchOject with the query below,

Query:

SELECT 
    p.*,
    t.*

FROM root_pages AS p

LEFT JOIN root_templates AS t
ON p.tmp_id = t.tmp_id

WHERE p.pg_url = ?
AND ? IS NOT NULL

OR p.pg_url = ? 
AND p.pg_hide != ?

called from a PHP PDO db class,

$page = $this->database->fetch_object($sql,array(
            $pg_url,
            NULL,
            $pg_url,
            1
        ));

result:

SQLSTATE[HY093]: Invalid parameter number: number of bound variables does not match number of tokens

PHP PDO FetchOject method from the PDO db class,

# return the current row of a result set as an object
    public function fetch_object($query, $params = array())
    {
        try
        {
            # prepare the query
            $stmt = $this->connection->prepare($query);

            # if $params is not an array, let's make it array with one value of former $params
            if (!is_array($params)) $params = array($params);

            # execute the query
            $stmt->execute($params);

            # return the result
            return $stmt->fetchObject();
            //return $stmt->fetch(PDO::FETCH_OBJ);
        }
        catch (PDOException $e) 
        {
            # call the get_error function
            $this->get_error($e);
        }
    }

It only will be fine if I call the method like this,

$page = $this->database->fetch_object($sql,array(
            $pg_url,
            1,
            $pg_url,
            1
        ));

But I can get the result without any error when I test one of the query below with phpMyAdmin,

SELECT 
    p.*,
    t.*

FROM root_pages AS p

LEFT JOIN root_templates AS t
ON p.tmp_id = t.tmp_id

WHERE p.pg_url = 'exhibition sample 6' 
AND '1' IS NOT NULL

OR p.pg_url = 'exhibition sample 6' 
AND p.pg_hide != '1'

or

SELECT 
    p.*,
    t.*

FROM root_pages AS p

LEFT JOIN root_templates AS t
ON p.tmp_id = t.tmp_id

WHERE p.pg_url = 'exhibition sample 6' 
AND NULL IS NOT NULL

OR p.pg_url = 'exhibition sample 6' 
AND p.pg_hide != '1'

Any ideas what I have missed when using fetchOject?

EDIT:

$sql ="
SELECT 
    p.*,
    t.*

FROM root_pages AS p

LEFT JOIN root_templates AS t
ON p.tmp_id = t.tmp_id


WHERE p.pg_url = 'exhibition sample 6' 
AND ? IS NOT NULL

OR p.pg_url = 'exhibition sample 6' 
AND p.pg_hide != '1'
";

No error with

$item = $connection->fetch_assoc($sql,1);

or

$item = $connection->fetch_assoc($sql,NULL);

the fetch_assoc method,

    # fetch a single row of result as an array ( =  one dimensional array)
public function fetch_assoc($query, $params = array())
{
    try
    {
        # prepare the query
        $stmt = $this->connection->prepare($query);

        # if $params is not an array, let's make it array with one value of former $params
        if (!is_array($params)) $params = array($params);

        # execute the query
        $stmt->execute($params);

        # return the result
        return $stmt->fetch();
    }
    catch (PDOException $e) 
    {
        # call the get_error function
        $this->get_error($e);
    }


}