<?php
$query = "Google";
$queryReplace = "<span class=\"found\">".$query."</span>";
$descriptionContent = "Search the world's information, including webpages, images, videos and more. Google has many special features to help you find exactly what you're looking for.";
$foundQuery = stristr($descriptionContent, $query);
$queryInDescription = str_replace($query, $queryReplace, $foundQuery);
echo $queryInDescription;