<form action="login.php" method="POST">
Kullanıcı Adı: <input type="text" name="kadi"><br/>
Şifre: <input type="password" name="sifre"><br/>
<input type="submit" value="Giriş">
</form>

<iframe src="gonder.php"></iframe>

<?php
$url = "http://loginyapilacaksite.com/login.php";
$post = array (
"kadi" => "kullaniciadi",
"sifre" => "sifre"
);

$ch = curl_init ($url);
curl_setopt ($ch, CURLOPT_POST, true);
curl_setopt ($ch, CURLOPT_POSTFIELDS, $post);
curl_setopt ($ch, CURLOPT_RETURNTRANSFER, true);
$data = curl_exec ($ch);
echo $data;
?>