I use php inside of var.js file with this .htaccess.
<Files var.js>
AddType application/x-httpd-php .js
</Files>
Then I write php code in the .js file
<?php
// This is a `.js` file but works with php
echo "var js_variable = '$php_variable';";
When I got the MIME type warning on Chrome, I fixed it by adding a Content-Type header line in the .js(but php) file.
<?php
header('Content-Type: application/javascript'); // <- Add this line
// This is a `.js` file but works with php
...
A browser won't execute .js file because apache sends the Content-Type header of the file as application/x-httpd-php that is defined in .htaccess. That's a security reason. But apache won't execute php as far as htaccess commands the impersonation, it's necessary. So we need to overwrite apache's Content-Type header with the php function header(). I guess that apache stops sending its own header when php sends it instead of apache before.