If you wanted a directory - or indeed the whole of your account - to parse all of your PHP files with the PHP 5 parser then all you need to do is to add a simple line to your .htaccess file:
AddHandler application/x-httpd-php5 .php .php4 .php3 .phtml
If you want a single file to be parsed by PHP5, then simply rename file from .php to .php5 and it will be parsed by PHP5