save-custom-field-to-the-database.php

<?php
/* Save the input to post_meta_data */

function jpen_save_custom_post_order( $post_id ){
  if ( !isset( $_POST['jpen_custom_post_order_nonce'] ) || !wp_verify_nonce( $_POST['jpen_custom_post_order_nonce'], basename( __FILE__ ) ) ){
    return;
  } 
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ){
    return;
  }
  if ( ! current_user_can( 'edit_post', $post_id ) ){
    return;
  }
  if ( isset( $_REQUEST['pos'] ) ) {
    update_post_meta( $post_id, '_custom_post_order', sanitize_text_field( $_POST['pos'] ) );
  }
}
add_action( 'save_post', 'jpen_save_custom_post_order' );
?>