$search_keyword = '';
if(!empty($_POST['search']['keyword'])) {
    $search_keyword = $_POST['search']['keyword'];
}
$sql = 'SELECT * FROM posts WHERE post_title LIKE :keyword OR description LIKE :keyword OR post_at LIKE :keyword ORDER BY id DESC ';
...
...
$pdo_statement = $pdo_conn->prepare($query);
$pdo_statement->bindValue(':keyword', '%' . $search_keyword . '%', PDO::PARAM_STR);
$pdo_statement->execute();
$result = $pdo_statement->fetchAll();