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

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

?>
</select>

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