Hi,
I would use the extention .php for this file; this way it will be treated as php and leave no 'trace' when someone manages to browse for this file over the web.
Then,.. In the page that you woul dlike to use the functions, you need to include the file through:
[man]include[/man]
[man]include_one[/man]
[man]require[/man] or
[man]require_once[/man]
e.g.,:
<?php
require_once("Path_to_file/function.php");
$result = myfunction();
?>