A string which would evaluate to a negative integer (such as "-123")
will not be turned into an integer when used as an array key. I don't
recall seeing anywhere an explicit explanation of how strings are
treated when used as array keys, but obviously those strings that
are valid representations of non-negative integers are implicitly
treated as integers, and everything else is seen as strings.
I think it would make sense to extend this to _all_ valid integers, as
the behavior in this example is not quite what one would expect:

$arr = array(-1 => "a", 0 => "b", 1 => "c");
$arr["-1"] = "e";
$arr["0"] = "f";
$arr["1"] = "g";
print_r($arr);
echo "\n";
var_dump($arr);