Fatal parse errors created in Php version 4

When using Php 4, the following fatal error is generated as soon as you enable the module:

Parse error: parse error, unexpected '=', expecting ')'  in ... (referring to several lines in the htmltidy.module)

It turns out that the problem is that Php 4 chokes completely when you try to assign a default value for a variable that is passed by reference. In Php 5 everything works fine. It looks like there is really no need to set a default value in the current code, so removing it makes things work again in Php 4.

See an explanation of the problem at http://www.php.net/manual/en/language.references.pass.php