Strongly Recommended to Upgrade to PHP 5.6, 7.0, or 7.1
Because the PHP Group stated that PHP 5.5 is no longer supported, there won’t be any more updates to that version, including security fixes. To avoid the potential for security issues on App Service, we plan to retire our support of PHP 5.5 in January 2018. Currently, we support PHP 5.6, 7.0 and 7.1.
This retirement will impact all customers who are running their Web App on the PHP 5.5 runtime (unless you are using a custom PHP runtime). We strongly recommend that you upgrade to a supported version of PHP because your Web App will be auto-moved to PHP 5.6. For more information on how to migrate from PHP 5.5 to another version of PHP, please review the documentation on the Appendices webpage on PHP.net.
Although we’re removing the PHP 5.5 runtime from the platform, we’re aware that some applications are complex and may be tied to a specific PHP runtime version. In order to continue to support your PHP 5.5 applications, it’s possible to remain on PHP 5.5 runtime. For more information, please visit the “How to: Use a custom PHP runtime” section of the Configure PHP in Azure App Service Web Apps documentation webpage.