In our software, we used . phtml File Extension to allow freedom of customizing the design layout, theme, blocks and template as you wish. It's an alternative file extension for a PHP generated page. It could contain HTML or XHTML; it depends on what the PHP has generated. Web page that contains PHP code; parsed by a PHP engine on the Web server, which dynamically generates HTML; often used for database access, such as retrieving data from a MySQL database; can be run using Apache or Windows Server with the PHP engine installed.
PHP pages typically use the .PHP file extension which contains core Business logic.