<?php
$url = 'http://your_domain.com/silent_post';
$data = array(
    'x_response_code' => '1',
    'x_response_subcode' => '1',
    'x_response_reason_code' => '1',
    'x_response_reason_text' => 'This transaction has been approved.',
    'x_auth_code' => '',
    'x_avs_code' => 'P',
    'x_trans_id' => '1821199455',
    'x_invoice_num' => '',
    'x_description' => '',
    'x_amount' => '9.95',
    'x_method' => 'CC',
    'x_type' => 'auth_capture',
    'x_cust_id' => '1',
    'x_first_name' => 'John',
    'x_last_name' => 'Smith',
    'x_company' => '',
    'x_address' => '',
    'x_city' => '',
    'x_state' => '',
    'x_zip' => '',
    'x_country' => '',
    'x_phone' => '',
    'x_fax' => '',
    'x_email' => '',
    'x_ship_to_first_name' => '',
    'x_ship_to_last_name' => '',
    'x_ship_to_company' => '',
    'x_ship_to_address' => '',
    'x_ship_to_city' => '',
    'x_ship_to_state' => '',
    'x_ship_to_zip' => '',
    'x_ship_to_country' => '',
    'x_tax' => '0.0000',
    'x_duty' => '0.0000',
    'x_freight' => '0.0000',
    'x_tax_exempt' => 'FALSE',
    'x_po_num' => '',
    'x_MD5_Hash' => 'A375D35004547A91EE3B7AFA40B1E727',
    'x_cavv_response' => '',
    'x_test_request' => 'false',
    'x_subscription_id' => '365314',
    'x_subscription_paynum' => '1',
);

$parameters = array(
    'http' => array(
        'method' => 'POST',
        'header'  => 'Content-type: application/x-www-form-urlencoded' . "\r\n",
        'content' => http_build_query($data),
    )
);

$start = microtime(true);
$pointer = fopen($url, 'rb', false, stream_context_create($parameters));
if (!$pointer) {
    die('Cannot open URL.');
}

$response = stream_get_contents($pointer);

fclose($pointer);

$elapsed = microtime(true) - $start;

if ($response === false) {
    die('Cannot read from URL');
}

echo 'Request took ' . round($elapsed, 2) . ' seconds. ';
echo 'Response (' . strlen(utf8_decode($response)) . ' Bytes) was:<hr/>' . $response;