PHP Redirect with POST data

Question

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");
  }
}

$_SESSION is your friend if you don't want to mess with Javascript

Let's say you're trying to pass an email:

On page A:

// Start the session
session_start();

// Set session variables
$_SESSION["email"] = "[email protected]";

header('Location: page_b.php');

And on Page B:

// Start the session
session_start();

// Show me the session!  
echo "<pre>";
print_r($_SESSION);
echo "</pre>";

To destroy the session

unset($_SESSION['email']);
session_destroy();

I have another solution that makes this possible. It requires the client be running Javascript (which I think is a fair requirement these days).

Simply use an AJAX request on Page A to go and generate your invoice number and customer details in the background (your previous Page B), then once the request gets returned successfully with the correct information - simply complete the form submission over to your payment gateway (Page C).

This will achieve your result of the user only clicking one button and proceeding to the payment gateway. Below is some pseudocode

HTML:

<form id="paymentForm" method="post" action="https://example.com">
  <input type="hidden" id="customInvoiceId" .... />
  <input type="hidden" .... />

  <input type="submit" id="submitButton" />
</form>

JS (using jQuery for convenience but trivial to make pure Javascript):

$('#submitButton').click(function(e) {
  e.preventDefault(); //This will prevent form from submitting

  //Do some stuff like build a list of things being purchased and customer details

  $.getJSON('setupOrder.php', {listOfProducts: products, customerDetails: details }, function(data) {
  if (!data.error) {
    $('#paymentForm #customInvoiceID').val(data.id);
    $('#paymentForm').submit();   //Send client to the payment processor
  }
});

I know this is an old question, but I have yet another alternative solution with jQuery:

var actionForm = $('<form>', {'action': 'nextpage.php', 'method': 'post'}).append($('<input>', {'name': 'action', 'value': 'delete', 'type': 'hidden'}), $('<input>', {'name': 'id', 'value': 'some_id', 'type': 'hidden'}));
actionForm.submit();

The above code uses jQuery to create a form tag, appending hidden fields as post fields, and submit it at last. The page will forward to the form target page with the POST data attached.

p.s. JavaScript & jQuery are required for this case. As suggested by the comments of the other answers, you can make use of <noscript> tag to create a standard HTML form in case JS is disabled.