You can use any function from inc/_core/_misc.funcs.php and some other files loaded by default

function some_hook( $params )
{
    echo maxwords($params['whatever'], 30);
}

If you are trying to use a function and get an error that it's unknown, you should include the file defining that function with "load_funcs"

function some_hook( $params )
{
    echo get_dirsize_recursive('/some/directory/'); // error: unknown function

    load_funcs('files/_file.funcs.php');

    echo get_dirsize_recursive('/some/directory/'); // prints directory size
}