By default RichTextEditor automatically render html code to support AJAX. You can turn it off by setting this property to "false" for non-Ajax pages.
Example
<?php
$rte=new RichTextEditor();
$rte->Text="Type here";
$rte->RenderSupportAjax=false;
$rte->ID="Editor1";
$rte->MvcInit();
echo $rte->GetString();
// use $_POST["Editor1"]to retrieve the data
?>