manufacturer with "&" sign in name is not filterable

Hi,

It indeed comes from the & in the name because you have the "redirect POST mode" setting activated and thus the parameters of the filters are passed in the URL (and the & in the URL are not interpreted correctly).
However, I'm not able to reproduce the issue on my end. That's because the value of brand filters are the ID of the brands, not the name.
So I don't see why it's using the name of the brand on your website. It shouldn't be possible... unless of course you're not using the brand of the product but a custom field. In that case, in your custom field, you have two field for each value:
- the value which is passed in the URL
- the title which is displayed to the user
So make sure you don't have the & in the value field and it will work fine.
Or you could turn off the redirect post mode.

Also, change the line:

$parameters[$name] = $_POST[$key];

$parameters[$name] = urlencode($_POST[$key]);