By default, PHP is set to announce its presence whenever anyone asks - this is usually through the web server. You can turn this functionality off by editing your php.ini file, and changing "expose_php" to "Off".
If you do this, as well as using a different file extension, your use of PHP is mostly hidden. However, if your code generates any error messages, your use of PHP will become immediately obvious. To get around this, and thereby truly hiding PHP, you should force PHP not to display error messages - edit your php.ini file and set "display_errors" to "Off".
This will make debugging a little harder, but be sure to set "log_errors" to "On" - this will make sure that whenever your script generates an error, it will be stored away in the error log file so that you can analyse the problem.