PHP Redirect with POST data

Generate a form on Page B with all the required data and action set to Page C and submit it with JavaScript on page load. Your data will be sent to Page C without much hassle to the user.

This is the only way to do it. A redirect is a 303 HTTP header that you can read up on http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html, but I'll quote some of it:

The response to the request can be found under a different URI and SHOULD be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource. The new URI is not a substitute reference for the originally requested resource. The 303 response MUST NOT be cached, but the response to the second (redirected) request might be cacheable.

The only way to achieve what you're doing is with a intermediate page that sends the user to Page C. Here's a small/simple snippet on how you can achieve that:

<form id="myForm" action="Page_C.php" method="post">
<?php
    foreach ($_POST as $a => $b) {
        echo '<input type="hidden" name="'.htmlentities($a).'" value="'.htmlentities($b).'">';
    }
?>
</form>
<script type="text/javascript">
    document.getElementById('myForm').submit();
</script>

You should also have a simple "confirm" form inside a noscript tag to make sure users without Javascript will be able to use your service.

/**
  * Redirect with POST data.
  *
  * @param string $url URL.
  * @param array $post_data POST data. Example: ['foo' => 'var', 'id' => 123]
  * @param array $headers Optional. Extra headers to send.
  */
public function redirect_post($url, array $data, array $headers = null) {
  $params = [
    'http' => [
      'method' => 'POST',
      'content' => http_build_query($data)
    ]
  ];

  if (!is_null($headers)) {
    $params['http']['header'] = '';
    foreach ($headers as $k => $v) {
      $params['http']['header'] .= "$k: $v\n";
    }
  }

  $ctx = stream_context_create($params);
  $fp = @fopen($url, 'rb', false, $ctx);

  if ($fp) {
    echo @stream_get_contents($fp);
    die();
  } else {
    // Error
    throw new Exception("Error loading '$url', $php_errormsg");
  }
}