Código PHP:
<?php
$region=$_POST['region'];
$numero=$_POST['numero'];
$texto=$_POST['mensaje'];
//Obtener claves
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'http://slipsms.com/send.php');
curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)');
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Accept-Language: es-es,en"));
curl_setopt($ch, CURLOPT_TIMEOUT, 10);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION,0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$result = curl_exec($ch);
$error = curl_error($ch);
curl_close($ch);
preg_match_all("(<input type=\'tel\' name=\'(.*)\' id=\'tel\' value=\'\' class=\'text\' />)siU", $result, $matches1);
preg_match_all("(<input type=\'hidden\' name=\'(.*)\' id=\'tell\' value=\'\' class=\'texta\' />)iU", $result, $matches2);
preg_match_all("(<input type=\"hidden\" name=\"(.*)\" value=\"(.*)\">)iU", $result, $matches3);
$return = trim($matches1[1][0]);
$key = trim($matches2[1][0]);
$key2 = trim($matches3[1][0]);
$key3 = trim($matches3[2][0]);
//Iniciar sesión
$parametros_post = 'country='.urlencode($region).'&'.$return.'='.urlencode('').'&'.$key.'='.urlencode($region.$numero).'&'.$key2.'='.urlencode($key3).'&messages='.urlencode($texto);
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'http://slipsms.com/message.php');
curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)');
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Accept-Language: es-es,en"));
curl_setopt($ch, CURLOPT_POST,true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $parametros_post);
curl_setopt($ch, CURLOPT_HEADER, false);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_close($ch);
?>