<?php
    function selected($founders, $id) {
        foreach ($founders as $founder) {
                        $selected = ($founder == $id) ? ' selected' : '';
                        $options[] = '<option value="'. $founder. '" '. $selected .'>';
        }
                return implode('', $options);
    }
?>
<select class="select2" name="founder_id[]" multiple="multiple">
<?php
        $founders = explode(",",$company->founder_id);

        // print_r($founder);
        $stmt = $person->read();
        $rowS = $stmt->fetch(PDO::FETCH_ASSOC);
        // print_r($row);
        foreach ($rows as $row){
                echo $row[$id];
                echo selected($founders, $row[$id]);
                echo $row[$id]."</option>";
        }

?>
</select>

In above $company->founder_id = 1,2,3,4