Ok, I finally got to fix the library... I explained everything here (you can also download the php-openid library after my changes).
I needed to do what Paul Tarjan suggested but, also, I needed to modify the Auth_OpenID_detectMathLibrary and add the static keyword to a lot of functions. After that It seems to work perfectly although it is not an ideal solution... I think that someone should rewrite the whole library in PHP 5...
