Show notice when WPLT is not in mu-plugins

[joeguilmette]

I tried this:

$mu_plugins = 'false';
// check if plugin is in mu-plugins
function my_plugin_override() {
    $mu_plugins = 'true';
}
add_action( 'muplugins_loaded', 'my_plugin_override' );

if ($mu_plugins == 'false') {
    function wplt_not_in_muplugins() {
        ?>
<div class="notice notice-info is-dismissible">
<h2><?php _e( 'NOPE', 'wp-local-toolbox' ); ?></h2>
<p><?php _e( "WP Local Toolbox is configured with constants added to your wp-config.php file.", 'wp-local-toolbox' ); ?></p>
<p><?php _e( "Read more: ", 'wp-local-toolbox' ); ?><a href="https://github.com/joeguilmette/wp-local-toolbox" style="top:0">https://github.com/joeguilmette/wp-local-toolbox</a></p>
</div>
        <?php
    }
    add_action( 'admin_notices', 'wplt_not_in_muplugins' );
}
echo $mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins.$mu_plugins;

But couldn't get it to work.