PDO how to check if input empty to not use WHERE

Hello I would like how I can transform the following code to use BindValues and to do the same job.

$sql = "SELECT * FROM documents WHERE 1";
            if (!empty($topic))   { $sql .= " AND topic = '$topic' ";   }
            if (!empty($date ))   { $sql .= " AND date LIKE '%$date%' "; }

WHERE clause to be applied only on the filled input So my question is how to transform this code in to the the following query

$sql = new page($pages "SELECT * FROM table WHERE topic LIKE :topic OR date LIKE :date ", $option);

$sql ->BindValue(':topic ', $_POST['topic '], PDO::PARAM_STR);
$sql ->BindValue(':date ', $_POST['date  '], PDO::PARAM_STR);