var input_value = $(this).val();
var new_value = input_value + " some_text";
$(this).val(new_value);
<ul>
<? foreach ($menu as $item) { ?>
<li>
<a href="<?= $item->link ?>"><?= $item-title ?></a>
</li>
<? } ?>
</ul>
<ul>
<? $current_url = add_query_arg($wp->query_string, "", home_url( $wp->request)) ?>
<? foreach ($menu as $item) { ?>
<li <?= $current_url == $item->link ? "class='current'" : ""?>
<a href="<?= $item->link ?>"><?= $item-title ?></a>
</li>
<? } ?>
</ul>
<input type="hidden" name="data" value="<?=data?>" />
var data = $('input[name="data"]').val();
$('#load_content').load("/app/views/admin/XXXXXXX.php?data="+data);
$data = $_GET['data'];