<?php
$paragraph = "<p>With natural, refreshing scents and stimulating formulas, here you’ll find just what you need to experience a new level of comfort and luxury</p>";
var_dump(preg_replace('/<\/p>$/', '<em>EMEMEMEMEMEM</em></p>', $paragraph));
string(171) "<p>With natural, refreshing scents and stimulating formulas, here you’ll find just what you need to experience a new level of comfort and luxury<em>EMEMEMEMEMEM</em></p>"