Installation:requirements
From PrometheusIPN Wiki
WEBSITE REQUIREMENTS
- PHP 5.4 or greater
- A MySQL database
- pdo_mysql extension enabled in your php.ini
- cURL enabled in your php.ini
- mbstring
- php-xml
- OpenID / OpenSSL
I recommend using gameswitchers.co.uk for webhosting as they're quite cheap and definitely meet the Prometheus requirements
How to check if your webserver supports Prometheus
Download this and extract it on your webserver. Then visit the file called prometheus_check.php. It will now tell you if your PHP version can run Prometheus
Install instructions found here