$html = '<tr>
    <td>Some Marker I know<td>
    <td>This is what I want</td>
<tr>

preg_match_all( '#Some Marker I know<td><td>(.*?)</td>#', $html, $match );

$html = '<tr class='unknown another' id='no idea'>
    <td attribute='no idea' class='no idea'>Some Marker I know<td>
    <label>This is what I want</label>
<tr>