public function changeUdi()
 {
     $mCatalogos = new mCatalogos();
     $mCatalogos->nuevaUdi($_POST['valor']);
     echo "\$" . $_POST['valor'];
 }
<?php

include_once "../model/mCatalogos.php";
$mudis = new mCatalogos();
$mudis->nuevaUdi($_POST['valor']);
echo $_POST['valor'];