PDO: Invalid parameter number: mixed named and positional parameters

I have come across this warning I've not seen before:

Warning: PDOStatement::execute() [pdostatement.execute]: SQLSTATE[HY093]: Invalid parameter number: mixed named and positional parameters in...

Referring to the following PDO query (have simplified the function for ease of reading):

$offset = 0;
$limit = 12;
function retrieve_search_posts($searchfield, $offset, $limit){


        $where = array();

        $words = preg_split('/[\s]+/',$searchfield);

        array_unshift($words, '');
        unset($words[0]);

        $where_string = implode(" OR ", array_fill(0,count($words), "`post_title` LIKE ?"));

        $query = "
                                SELECT  p.post_id, post_year, post_desc, post_title, post_date, img_file_name, p.cat_id
                                FROM    mjbox_posts p
                                JOIN    mjbox_images i
                                ON      i.post_id = p.post_id
                                        AND i.cat_id = p.cat_id
                                        AND i.img_is_thumb = 1
                                        AND post_active = 1
                                WHERE $where_string
                                ORDER BY post_date
                                LIMIT :offset, :limit
                                DESC";
        $stmt = $dbh->prepare($query);

        foreach($words AS $index => $word){
            $stmt->bindValue($index, "%".$word."%", PDO::PARAM_STR);
        }
        $stmt->bindParam(':offset', $offset, PDO::PARAM_INT);
        $stmt->bindParam(':limit', $limit, PDO::PARAM_INT);
        $stmt->execute();

        $searcharray = $stmt->fetchAll(PDO::FETCH_ASSOC);

        return $searcharray;
    }

The function and PDO query works fine without the offset and limit variables included in the query. So what might be causing this warning?

Thanks

The error message has it pretty clear: you cannot mix named (:name) and positional (?) placeholders in one prepared statement, so you have to use only one type.

To fix the issue, instead of

LIMIT :offset, :limit

make it

LIMIT ?, ?

and change

$stmt->bindParam(':offset', $offset, PDO::PARAM_INT);
$stmt->bindParam(':limit', $limit, PDO::PARAM_INT);

to:

$stmt->bindValue($index+1, $offset, PDO::PARAM_INT);
$stmt->bindValue($index+2, $limit, PDO::PARAM_INT);