Because the PHP Group stated that PHP 5.5 is no longer supported, there will not be any more updates to that version, including security fixes. To avoid the potential for security issues on Azure 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 affect 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 automatically 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 are removing the PHP 5.5 runtime from the platform, we are aware that some applications are complex and might be tied to a specific PHP runtime version. To continue to support your PHP 5.5 applications, it is possible to remain on the PHP 5.5 runtime. For more information, see the “How to: Use a custom PHP runtime” section of the Configure PHP in Azure App Service Web Apps documentation webpage.