Undefined index SERVER_NAME

[malthert]

wp-includes/pluggable.php

<?php
$sitename = strtolower( $_SERVER['SERVER_NAME'] );

1) this requires an !empty check to avoid a notice
2) I suggest to change this to HTTP_HOST as this is more commonly correct (if not set) than SERVER_NAME (maybe fallback to SERVER_NAME)
3) this must be properly sanitized

[SergeyBiryukov]

Hi there, welcome back to WordPress Trac!

Thanks for the report, we're already tracking this issue in #25239.