On array_sum()'s page it's said :
"
Note: PHP versions prior to 4.0.6 modified the passed array itself and converted strings to numbers (which most of the time converted them to zero, depending on their value).
"
According to 2 user comments, two tests, and extract from the changelog of 4.2.1 (Fixed a couple of bugs in array_sum() and array_merge(). (Andrei))

So, the version should be changed to 4.2.1 (it works with 4.2.2)