Version 5.x System Requirements
From WHMCS Documentation
Most current web servers with PHP & MySQL installed will be capable of running WHMCS. However, the specific system requirements are:
- Windows or Linux OS
 - PHP Version 5.0.x - 5.4.x (with JSON)
 - MySQL Version 4.1.x and above (Strict Mode Disabled)
 - Curl Support (with SSL)
 - GD2 Image Library
 - Ioncube Loaders 4.x or later
 
Note: PHP Version 5.5 is not supported at this time.
If you would like any more information or need help in determining if your server meets these requirements, please contact us.