How to save values of a custom input field on the Network > Site Info screen

Question

I'm struggle with implementing a custom input field on the screen of a Site Info in a Network. I've managed to create a field with help of this post but i have problems with saving the value.

When i output the $_POST variable in the admin_footer-function, it is an empty array.

<?php
add_action('admin_footer', 'pg_custom_site_options');
function pg_custom_site_options(){

    global $pagenow;

    if( 'site-info.php' == $pagenow ) {

        global $details;

        $blog_id = isset( $_REQUEST['id'] ) ? intval( $_REQUEST['id'] ) : 0;
        if ( ! $blog_id ) wp_die( __('Invalid site ID.') );

        $saved_value = get_blog_option( $blog_id, 'blog_order');

        ?>
        <table>
            <tr id="user16975_custom_options">
                <th scope="row">Order</th>
                <td><input type="text" size="5" name="blog[blog_order]" value="<?php echo esc_attr( $saved_value ) ?>" ></td>
            </tr>
        </table>
        <script>
            jQuery(function($){
                $('.form-table tbody').append($('#user16975_custom_options'));
            });
        </script>
        <?php


        // DON'T USE IT - SEE MY THE ANSWER BEHIND
        // ---------------------------------------

        // Use a default value here if the field was not submitted.
        // $new_field_value = '0';

        //global $_POST; // Just a Test

        // echo var_dump($_POST); // Is an empty array :(

        // if ( !empty($_POST['blog']['blog_order']) ){
        //     $new_field_value = $_POST['blog']['blog_order'];
        // }else{
        //     echo "nothing to do here";
        // }

        // save option into the database
        // update_option( 'blog_order', $new_field_value);
    }
}
?>

The strange part is, all values are presented after submit / reload of the site in Chrome Developer Tools or Firebug.

Answer 1

The problem was a redirect in site-info.php. After that the $_POST variable was empty in the code inserting in the admin_footer.

I just need to use a further hook 'admin_init' and save the value here.

add_action('admin_init', 'pg_save_custom_site_options');
function pg_save_custom_site_options(){

    global $pagenow;

    if( 'site-info.php' == $pagenow &&
        isset($_REQUEST['action']) &&
        'update-site' == $_REQUEST['action']
    ) {
        // Use a default value here if the field was not submitted.
        $new_field_value = '0';

        if ( isset( $_POST['blog']['blog_order'] ) ) {
            $new_field_value = intval( $_POST['blog']['blog_order'] );

            // save option into the database
            if( is_int($new_field_value) ){
                update_blog_option( $_POST['id'], 'blog_order', $new_field_value );
            }

        }

    }
}