Embedded in an html file, code between tags is executed as PHP code. The result printed by that code is embedded in the HTML page. While the mapping of URLs to files is configurable in the web server, PHP files usually end with the php extension.
<html> <head><title></title></head> <body> <?php echo "Hello World"; ?> <p> Check out the source and see the PHP tags. </p> </body> </html>