Hi;

this feature being asked for quite often I think, here is a way to include username in a text (category description, [extension by plg] PWG Stuffs block...) :
copy following code into your Personnal Plugin thanks to LocalFiles Editor

// js_username_prefilter_trigger
// Adds a JavaScript variable named USERNAME in the header template,
// if this Smarty variable is set.
//
// no variables, no return
//
add_event_handler('init', 'js_username_prefilter_trigger');

function js_username_prefilter_trigger() {
  global $template;
  $template->set_prefilter('header', 'js_username_prefilter');
}

function js_username_prefilter($content, &$smarty) {
  return preg_replace(
    '/<\/head>/',
    '</head> <script type="text/javascript"> var USERNAME = "{if isset($USERNAME)}{$USERNAME}{/if}"; </script>',
    $content
  );
}

Once the Personnal Plugin activated in the plugins list, perform a compiled templates purge : using Piwigo 2.1.x, Admin > Tools > Maintenance > Purge compiled templates .

Now a text will be able to display the user name thanks to the bold part of following text :

Currently testing.<br />
<script type="text/javascript">
if (USERNAME != '') document.write('Hi, ' + USERNAME + '!<br />');
</script>
And here we are!

With this example, that I put in a PWG Stuffs block, one can see :

Currently testing.
And here we are!

when no user is currently identified;
and:

Currently testing.
Hi, Luc!
And here we are!

when user "Luc" is currently identified.

Demo:

as guest ;
identified as user "Piwigo" (thanks to [extension by LucMorizur] Event Cats (pardon for hidden advertising ;-) !)).
Do create other users to check that it works properly!

:-)