Here is a quote from ModernBill's site:
QUOTE
System Requirements
Unix/Linux OS (Windows OS Unofficially Supported)
SSL Certificate (HTTPS)
cURL with SSL Enabled
PHP Version 4.1.x or Greater
Database: MySQL
Zend Optimizer™ Version 2.1.0 or Greater
or
IonCube Runtime Decoder (Included)
As you know we support each feature from the list