The following table shows the minimum and recommended system requirements for running WHMCS v7.5 and later (this was correct as of 06/11/2018)
|PHP Version||5.6.0||Latest 7.1 or 7.2 Release|
|PHP Memory Limit||64MB||128MB**|
|PHP Database Extension||PDO||PDO|
|PHP Extensions||Curl with SSL, GD2 Image Library, JSON Support, XML||Curl with SSL, GD2 Image Library, JSON Support, IMAP, MBString, Iconv|
|Ioncube Loaders||10.1.0 or later||The latest 10.1.x version|
Looking for the requirements for WHMCS 7.0 – 7.4? Guide is available over at WHMCS here
* We validate WHMCS to run in Linux based environments running the Apache web server. Other environments such as Windows-based configurations may experience compatibility issues and are not supported.
** Memory requirements vary depending upon the size and volume of activity in an installation. Your exact requirements may differ.
*** As part of PCI compliance, we recommend a SSL library capable of TLS 1.1 and/or 1.2. If using OpenSSL, version 1.0.1c or newer is recommended.
This guide is correct as of 06/11/2018 for up to date article please go to WHMCS here