$string='<tr><td>abc</td><td>def</td><td>ghi</td></tr>';
echo $td_num = substr_count($string,'<td>');
for ($i=0;$i<$td_num;$i++)
{
echo "<br /><br />".$i."<br />";

$First_td[$i] = strpos($string,'<td>');
echo 'First posITion: '.$first_td[$i]."<br />"; 

$last_td[$i]=strpos($string,"</td>");
echo 'Last position: '.$last_td[$i]."<br />";

$td_value[$i]=substr($string,$first_td[$i],$last_td[$i]);

echo "TD Value: ".$td_value[$i]."<br />";   
echo $string_size = strlen($string)."<br />";

$string = substr($string,$last_td[$i]);
}

结果是：

3

       0
       First position: 4
       Last position: 11
       TD Value: abc45


       1
       First position: 5
       Last position: 0
       TD Value: 
       34


       2
       First position: 5
       Last position: 0 
       TD Value: 
       34

请告诉我,为什么它不起作用？我怎样才能使它工作？如何以不同方式解析TD值？