WHMCS runs on PHP code and requires several PHP extensions in order to function normally. If there is a problem with PHP on your server, you will experience problems in many areas of WHMCS. For successful installation, updates, and operation, your system must meet the minimum system requirements for your WHMCS version. ionCube Loader® is a PHP extension that allows decoding and encoding of WHMCS files. Make sure your system uses the correct version. Troubleshoot and resolve 500 Internal Server Error messages in WHMCS. Troubleshoot and resolve Allowed memory size of xxxxx bytes exhausted errors. You can add PHP code to your custom templates. We recommend using the hook system for most customizations. Troubleshoot and resolve Down for Maintenance errors. Troubleshoot and resolve ExceptionLoggingTrait Class Not Found errors. Troubleshoot and resolve A fatal error occurred while fetching the update errors. Troubleshoot and resolve Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 20480 bytes) errors. Troubleshoot and resolve issues with frequent logouts. Troubleshoot and resolve InvalidArgumentException: Data missing errors. Troubleshoot and resolve Site error: the ionCube PHP Loader needs to be installed. errors. Troubleshoot and resolve Legacy Smarty Tags errors. To promote better security, we plan to move to Smarty 4, which is not compatible with SmartyBC, in the near future. If you use certain Smarty tags, you must remove them now. Troubleshoot and resolve A license key is required. errors during installation. You can adjust the login session length for admins and users. WHMCS will automatically end login sessions after the set period of inactivity. Troubleshoot and resolve users logging out while downloading invoices. Troubleshoot and resolve Unable to find the socket transport errors while using Nominet for domain registrations. Troubleshoot and resolve warnings about the OPcache PHP extension.