Sending $_POST variable to another page using PHP without forms

Use cURL to send the POST from the server:

//set POST variables
$fields = array(
                  'secret_key' => urlencode('somevalue'),
                  'auth_key' => urlencode('othervalue'),
                  'charge_total' => 345
                );

//url-ify the data for the POST
foreach($fields as $key=>$value) { $fields_string .= $key.'='.$value.'&'; }
rtrim($fields_string, '&');

//open connection
$ch = curl_init();

//set the url, number of POST vars, POST data
curl_setopt($ch,CURLOPT_URL, 'https://www.yourgateway.com');
curl_setopt($ch,CURLOPT_POST, count($fields));
curl_setopt($ch,CURLOPT_POSTFIELDS, $fields_string);

//execute post
$result = curl_exec($ch);

//close connection
curl_close($ch);

See my update below.

I think you can use php/curl examples here: http://curl.haxx.se/libcurl/php/examples/, especially http://curl.haxx.se/libcurl/php/examples/simplepost.html

<?php
//
// A very simple PHP example that sends a HTTP POST to a remote site
//

$ch = curl_init();

curl_setopt($ch, CURLOPT_URL,"http://www.example.com/tester.phtml");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS,
            "postvar1=value1&postvar2=value2&postvar3=value3");

curl_exec ($ch);
curl_close ($ch); 
?>

It can be wrote into function likes this:

function sendPost($postURL, $post_string) {

// initiate curl object
$request = curl_init($postURL);

// set to 0 to eliminate header info from response
curl_setopt($request, CURLOPT_HEADER, 0);

// Returns response data instead of TRUE(1)
curl_setopt($request, CURLOPT_RETURNTRANSFER, 1);

// use HTTP POST to send form data
curl_setopt($request, CURLOPT_POSTFIELDS, $post_string);

// execute curl post and store results in $post_response
$post_response = curl_exec($request);

// Checks for Errors if needed
if (curl_errno($request) > 0)
{
        $_SESSION['curl_error'] = curl_getinfo($request);
}

curl_close ($request);

return $post_response;

}

$postURL is URL of your payment gateway

$post_string is your variables which match with format of payment gateway.

and you can parse the $post_response and take it values to show status to user. Of course, you know how to use var_dump or echo to see values of $post_response already