Checkbox CSV Metabox

<?php
add_action( 'admin_init', 'hhs_add_metaboxes' );
function hhs_add_metaboxes() {
  add_meta_box('hhs_checkbox_metabox', 'Checkboxes as CSV', 'hhs_checkbox_metabox', 'post', 'side', 'core');
}

function hhs_checkbox_metabox() {
    global $post;

    wp_nonce_field( plugin_basename( __FILE__ ), 'hhs_metabox_nonce' );

    $checkboxes = get_post_meta( $post->ID, 'checkboxes', true );

    // this is just a check to see the CSV
    echo "<p>CSV: $checkboxes</p>";

    $checkboxes = explode( ',', $checkboxes );

    echo '<p><strong>Checkboxes:</strong></p>
    <p>';

    for ( $i = 1; $i < 7; $i++ ) {
        echo '<input type="checkbox" name="checkboxes[]" value="'.$i.'"';

        if ( in_array($i, $checkboxes) )
            echo ' checked="checked"';

        echo ' /> '.$i.'<br />';
    }

    echo '</p>';
}

// Save data from meta box
add_action('save_post', 'hhs_checkbox_metabox_save');
function hhs_checkbox_metabox_save($post_id) {
  // verify nonce
  if ( !wp_verify_nonce( $_POST['hhs_metabox_nonce'], plugin_basename( __FILE__ ) ) )
    return;

  // check autosave
  if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE)
    return;

  // check permissions
  if (!current_user_can('edit_post', $post_id))
    return;

    $old['checkboxes'] = get_post_meta( $post_id, 'checkboxes', true );
    $new['checkboxes'] = $_POST['checkboxes'];
    $new['checkboxes'] = implode( ',', $new['checkboxes'] );

    if ( $new['checkboxes'] && $new['checkboxes'] != $old['checkboxes'] ) {
      update_post_meta($post_id, 'checkboxes', $new['checkboxes']);
    } elseif ( '' == $new['checkboxes'] && $old['checkboxes'] ) {
      delete_post_meta($post_id, 'checkboxes', $old['checkboxes']);
    }
}
?>