http://captcha.byethost13.com/ 
username : captcha
pass : captcha

<?php

$url = "http://captcha.byethost13.com/?a=login";

$reffer = "http://captcha.byethost13.com/?a=login";
$post_fields = 'a=do_login&username=captcha&password=captcha&validation_number=310067'; //
$agent = "Mozilla/5.0 (Windows NT 6.1; rv:15.0) Gecko/20100101 Firefox/15.0";
$cookie_file_path = "cook.txt";
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_USERAGENT, $agent);
curl_setopt($ch, CURLOPT_REFERER, $reffer);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, $post_fields);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_COOKIE, $cookie_file_path);


$result = curl_exec($ch);
curl_close($ch);

echo $result;

?>

nội dung file cook.txt

# Netscape HTTP Cookie File
# http://www.netscape.com/newsref/std/cookie_spec.html
# This file was generated by libcurl! Edit at your own risk.

captcha.byethost13.com	TRUE / TRUE 135898038 PHPSESSID a4da276999f8e097402d8e1b64b67fa4