We can do it in different ways. Configuration data are sent by POST.
Some of them should be parsed before saving to file.

Perhaps, I will use var_export(). However, how to do it best?

1. Without a class / function.
Fields names have no prefix and are equal to $cfg array's indexes.
<input name="option_name" />

In every file of options form (e.g. main options, content options, e-
mail...) you insert:
if(file_put_contents('cfg/file_name.php', var_export($_POST,1), 2))
{
...
}

If you must parse some data:
$_POST['data'] = Clean($_POST['data']);

2. With a class / function.
Fields names may contain prefix (e.g. u_).
<input name="u_option_name" />

Saving data:
$o = new Config(...);
$o -> add('option_name', 'int');
$o -> add('option_name');
$o -> save('file_name'); //or: $o = null;

3. Method 2 with dynamic options forms:

$o = new Config(...);
$o -> add('option_name', 'int', 50);

No, it isn't good example. You can have textarea fields (even with
editor), text, radio (more choices for 1 option), checkbox, select
(with <option>s which can be generated by other functions, e.g.
category list).

4. Array (more code):

$array = array(
'opt1' => (int) $_POST['u_option']
);

The main configuration page has above 30 options. However, the method
with $o -> add(...) may be slower. What do you think about it? Which
choice is the best or maybe you have other solutions?