Please let me know whether what I should do.
Although I am using a certain template engine, the template for XHTML has the xml declaration at the top, and this will be judged to be an error by PhpStorm.
Is this nonavoidable by change or other means of a setup?
Since a template file may be performed directly, an extension is .php.
If it can do, I would like to write in this way, although a head line can also be written as follows.
<?php echo '<?xml version="1.0" encoding="', $header_encoding, '"?>'; ?>