How to replace the last HTML list item?

How do I replace the last li element in this string?

Before:

$var = 
   '<li>anything<li/>
    <li>anything<li/>
    <li>anything<li/>
    <li>anything<li/>
    <li>anything<li/>';

After:

$var = 
   '<li>anything<li/>
    <li>anything<li/>
    <li>anything<li/>
    <li>anything<li/>
    <li class="foo">anything<li/>';

I tried with preg_replace():

preg_replace('<li>', '<li class="foo">', $var);

But it isn't the expected output. How do I change the code to get the expected output?