Edit in index.php:

// Your Pay4Coins-Shop-ID
$p4c_shop_id = 00000;

// Your Pay4Coins-API-Key
$p4c_api_key = 'XXXXXXXXXX';

// Your API-URL (receive_data.php)
$yourAPIurl = 'https://pay4coins.com/SDK/receive_data.php';

Edit in receive_data.php:

// Your Pay4Coins-API-Key
$p4c_api_key = 'XXXXXXXXXX';