PHP: need this exact position of string with a wildcard

First of all, thank you for reading this post and trying to help me. I am pretty new to php and i have some struggle.

I have created a filter in wordpress which should check two conditions. the filter is working fine however i have ran it some problems.

this is my code

     function em_validate($result, $EM_Event){

        if ($_REQUEST['em_tickets'] > 1 && $_REQUEST['booking_comment'] == ''){
        $EM_Event->add_error('Something');  
        $result = false; 
     }
     return $result;
    }

add_filter('em_booking_validate','em_validate', 1, 2);

This filter works fine, however the content of $_REQUEST['em_tickets']) is something like

<p><strong>23</strong></p><ul><li>5</li></ul><p></p>

can you come with some suggestions how i can read whats inbetween <li> and </li>

thank you for reading this post and coming with suggestions

via Chebli Mohamed