Assuming you thought to test using a string that's all digits. You have to know about PHP's brokenness to even think to check that. In a sane language, it doesn't matter what the contents of the strings are, they're just strings.

>Unit tests would have caught this, if you wrote them.

If I hadn't considered string-number-keys in my code, I would also have forgotten about them in my unit tests. Maybe you're different.

My unit tests did not cover all possible strings, unfortunately.