Use PHP to removed nested HTML list elements

I need a simple PHP function to change all nested list elements within a text string into one list. There may be multiple lists in the same string, each with several levels. I expect preg_replace() or something similar to be used. For example, all items in this list should be reduced to only one level: <ul> <li><strong>Level one</strong> <ul> <li class="large">Level two</li> <li>Level two <ul> <li>Level three</li> </ul> </li> </ul> </li> <li>Level one</li> </ul> Desired output: <ul> <li><strong>Level one</strong></li> <li class="large">Level two</li> <li>Level two</li> <li>Level three</li> <li>Level one</li> </ul> I would like this done within a couple of hours. Please confirm in your bid. Thank you.