wrap-last-word.function.php

<?php
/**
 * Wrap last word with span
 * @author: Elron
 * https://stackoverflow.com/questions/18612872/get-the-last-word-of-a-string
 */
function wrap_last_word($string) {
    // Breaks string to pieces
    $pieces = explode(" ", $string);

    // Modifies the last word
    $pieces[count($pieces)-1] = '<span class="is-last-word">' . $pieces[count($pieces)-1] . '</span>';

    // Returns the glued pieces
    return implode(" ", $pieces);
}

wrap_last_word('hello this is wrapped');
// returns this:
// hello this is <span class="is-last-word">wrapped</span>