Call to undefined function bi_to_str() after upgrading php from 5.2 to 5.5

Question

After upgrading php to 5.5, the following error occurs:

PHP Fatal error: Call to undefined function bi_to_str() in /var/www/index.php

I know that it's not a coding error, but it could be a missing library. But could an empty value cause that error? Is there an easy way to load that library using apt-get in Debian?

Answer 1

Well with the help I received in the comments, I was able to come up with a solution.
It seems that it's just the function that is missing. I was able to create a file called big_int.php, fill it with the contents found here , and just link to it using require_once in index.php, like this:

require_once('big_int.php');

All is well. Many thanks to Barmar for the helpful tip.