<?php
$m_shop = '924187188';
$m_orderid = '1';
$m_amount = number_format(100, 2, '.', '');
$m_curr = 'USD';
$m_desc = base64_encode('Test');
$m_key = 'секретный ключ';

$arHash = array(
 $m_shop,
 $m_orderid,
 $m_amount,
 $m_curr,
 $m_desc
);


$arParams = array(
 'reference' => array(
  'steamid' => '12345' // хочу это передать дальше дальше
 )
);

$key = md5('ключ для шифрования доп данных'.$m_orderid);

$m_params = base64_encode(openssl_encrypt(json_encode($arParams), 'AES-256-CBC', $key, OPENSSL_RAW_DATA));

$arHash[] = $m_params;


$arHash[] = $m_key;

$sign = "D107ECFD2095C0127E01C7AF1A172B3392DEADB300C16927AFC3BF86035D7AD5";
?>
<form method="post" action="https://payeer.com/merchant/">
<input type="hidden" name="m_shop" value="<?=$m_shop?>">
<input type="hidden" name="m_orderid" value="<?=$m_orderid?>">
<input type="hidden" name="m_amount" value="<?=$m_amount?>">
<input type="hidden" name="m_curr" value="<?=$m_curr?>">
<input type="hidden" name="m_desc" value="<?=$m_desc?>">
<input type="hidden" name="m_sign" value="<?=$sign?>">

<input type="hidden" name="form[ps]" value="2609">
<input type="hidden" name="form[curr[2609]]" value="USD">

<input type="hidden" name="m_params" value="<?=$m_params?>">
<input type="hidden" name="m_cipher_method" value="AES-256-CBC">

<input type="submit" name="m_process" value="send" />
</form>