$digital_fee = (isset ($_POST['digital_fee']) ? $_POST['digital_fee'] : '');
$banner_1 = (isset ($_POST['banner_1']) ? 1 : 0);

$statement = $db->prepare("INSERT INTO $table
                            (digital_fee, banner_1)
                            VALUES
                            (:digital_fee, :banner_1)
                            ON DUPLICATE KEY UPDATE
                            digital_fee=:digital_fee, banner_1=:banner_1");

$statement->bindParam(':arn_mkt_stn', $arn_mkt_stn);
$statement->bindParam(':digital_fee', $digital_fee);

$statement->execute();

// example function to handle the PDO named params db inserts

$digital_fee ='500';
$banner_1 = '0';

$arr_digital = array("digital_fee"=>$digital_fee, "banner_1"=>$banner_1);
$table = 'myTable';

//create sql string with $vars
$sql = "INSERT INTO $table (";
foreach ($arr_digital as $key => $val){
    $sql.= "$key, ";
}

$sql.= ") VALUES ( ";
foreach ($arr_digital as $key => $val){
    $sql.= ":$key, ";
}

$sql.= ") ON DUPLICATE KEY UPDATE ";
foreach ($arr_digital as $key => $val){
    $sql.= "$key=:$key, ";
}
echo $sql;

$statement = $db->prepare($sql);


$statement->bindParam(':arn_mkt_stn', $arn_mkt_stn);
foreach ($arr_digital as $key => $val){
         $statement->bindParam(":$key, $val");
}

$statement->execute();

Fatal error:
  Uncaught exception 'PDOException' with message
    'SQLSTATE[42000]: Syntax error or access violation: 1064
     You have an error in your SQL syntax; check the manual that corresponds
     to your MySQL server version for the right syntax to use near
     ') VALUES ( '500', '0', ) ON DUPLICATE KEY UPDATE digital_fee='500', banner_1='0'' at line 1'

$digital_fee ='500';
    $banner_1 = '0';

    $arr_digital = array("digital_fee"=>$digital_fee, "banner_1"=>$banner_1);
    $table = 'myTable';

    //create sql string with $vars
    foreach ($arr_digital as $key => $val){
        $columns[]= $key;
        $vals [] = ':'.$key;
        $updates[]= "$key = :$key";

    }
    $column = implode(', ', $columns);
    $values = implode(', ', $vals);
    $update = implode(', ', $updates);

    $sql = "INSERT INTO $table ($column) VALUES ($values) ON DUPLICATE KEY UPDATE $update";
    echo $sql;

    $statement = $db->prepare($sql);


    $statement->bindParam(':arn_mkt_stn', $arn_mkt_stn);
    foreach ($arr_digital as $key => $val){
            $statement->bindParam(":$key", $val);
    }

    $statement->execute();

$comma = "";
$sql = "INSERT INTO $table (";
foreach ($arr_digital as $key => $val){
    $sql.= $comma . "$key ";
    $comma = ","
}

$comma = "";
$sql.= ") VALUES ( ";
foreach ($arr_digital as $key => $val){
    $sql.= $comma . ":$key ";
    $comma = ","
}