$sel=".txt";
$content = "new link text";

$html = str_get_html("<li><a href='http://www.mylink.com' > <span class='txt'>link text</span>");

foreach ($html -> find($sel) as $element) {
    $element -> innertext = $content;
}

echo "element = " . htmlspecialchars($element)."<br>";
echo "html = " . htmlspecialchars($html)."<br>";

element = <span class='txt'>new link text</span>
html    =  <li><a href='http://www.mylink.com' > <span class='txt'>link text</span></a></li>