A WordPress plugin that shows a small green label during local development.

wp-local-flagging.php

<?php
/*
Plugin Name: Local Flagging
Plugin URI:
Description: Shows a flag on the page when the site is detected to be local.
Version: 1.5
Author: Matt van Andel
Author URI:
License: GPL2
Text Domain:
Domain Path:
Place in mu-plugins on your local environment and ensure that you have the following added to your local vhost...
    Header Set LOCAL-ENV true
    SetEnv LOCAL-ENV true
*/
if ( isset( $_SERVER['LOCAL-ENV'] ) ) {
	function maybe_output_local_flag() {
		echo '<div style="background:green;color:white;font-weight:bold;position:fixed;bottom:0;right:0;padding:5px 8px;z-index:999;font-family:sans-serif;">LOCAL</div>';
	}
	add_action( 'wp_footer', 'maybe_output_local_flag' );
	add_action( 'admin_footer', 'maybe_output_local_flag' );
	add_action( 'login_footer', 'maybe_output_local_flag' );
}