Use strpos to search and then highlight the matched letters

I want to perform a search on the $search string and then display the entire $search string with the matching letters highlighted.

This is the code so far:

//The user input
$user_input= $_POST['user_input'];

//The search
$search = 'Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas.'; 

$match = strpos($search, $user_input);
echo '$match';

?>