PHP Web爬网程序返回“未识别的偏移量1”错误

I have this script that i made trying to extract links and text from a site and post it in my page. although it does the work like a charm but it also shows a error on the top as unidentified offset 1. help me riddle it out..

code as follows:

<?php

function get_data($url) {

    $ch = curl_init();

    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);

    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

    curl_setopt($ch, CURLOPT_URL,$url);

    $result=curl_exec($ch);

    curl_close($ch);

    return $result;

}

$returned_content = get_data('http://www.usmle-forums.com/usmle-step-1-forum/');

$first_step = explode( '<tbody id="threadbits_forum_26">' , $returned_content );

$second_step = explode('</tbody>', $first_step[1]);

$third_step = explode('<tr>', $second_step[0]);

// print_r($third_step);

foreach ($third_step as $element) {

    $child_first = explode( '<td class="alt1"' , $element );

    $child_second = explode( '</td>' , $child_first[1] );

    $child_third = explode( '<a href=' , $child_second[0] );

    $child_fourth = explode( '</a>' , $child_third[1] );

    echo $final = "<a href=".$child_fourth[0]."</a></br>";

}

?>

error at line $child_second = explode( '</td>' , $child_first[1] ); and line $child_fourth = explode( '</a>' , $child_third[1] );

any help is appreciated..