[] IS the append operator. And += only works right if both sides are arrays. If not the right side will be converted. += is really intended for hashmaps, not integer indexes, for those use [].
+= does union-by-key: any keys present in the right array, and not in the left, are appended to the left.
`array_merge` will append all numeric keys from the right array to the left, under new key values, as if you used [] one-by-one; for string keys, all keys from the right are copied into the left, possibly overwriting what was there.
In real code, I either have all-numeric or all-string keys, and `array_merge` does what I want in bulk operations: it's essentially equal to Python list.extend and dict.update, respectively. `+` on arrays is basically useless for me.
