<?php
$start_string = '<!-- Page get start -->';

$end_string = '<!-- Page get end -->';

$ch = curl_init("http://www.site.com/forums/downloads.php?do=cat&id=12");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_HEADER, 0);

$res = curl_exec($ch);
curl_close($ch);

$res = explode($start_string, $res);
$res = explode($end_string, $res[1]);
$res = reset($res);

echo $res;
?>