assing value to hidden field pre submittion

functions.php

add_filter("gform_pre_submission_1", "obtainsumvaluesfield", 1);
function obtainsumvaluesfield($form)
{
    // get all entries in this form
    $entry = GFFormsModel::get_current_lead();

    $all = 0;
    foreach ($form["fields"] as &$field)
        if ($field->type == 'checkbox') {

            $field_value = $field->get_value_export($entry, $field->id);

            $values = explode(', ', $field_value);

            $all += array_sum($values);
        }

    $_POST["input_21"] = $all;
    return $form;
}