Código PHP:
<?php
$curl = curl_init();
curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, FALSE);
curl_setopt($curl, CURLOPT_HEADER, 0);
curl_setopt($curl, CURLOPT_POST, true);
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
curl_setopt($curl, CURLOPT_USERAGENT, "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)");
curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($curl, CURLOPT_COOKIEFILE, "cookiefile.txt");
curl_setopt($curl, CURLOPT_COOKIEJAR, "cookiefile.txt");
curl_setopt($curl, CURLOPT_URL, "http://www.dominio.com/login.php");
curl_setopt($curl, CURLOPT_POSTFIELDS, "un=usuario&pw=clave&");
$login = curl_exec ($curl);
curl_close ($curl);
echo $login;
?>