the documentation claims this behavior was changed after 4.0.6, but it appears to behave the same in 4.1.0. (and i don't see a commit to ext/standard/array.c that indicates any such change to array_sum.)

$a = array(1, 2, "foo");
print_r($a);
echo array_sum($a);
print_r($a);

results in:

Array
(
    [0] => 1
    [1] => 2
    [2] => foo
)
3Array
(
    [0] => 1
    [1] => 2
    [2] => 0
)