> arrays-that-are-arrays-sometimes-but-maps-other-times

god, the time i spent going paranoid debugging a stray "undefined array index 0" just to find out

  array_filter(
    is_uppercase,
    ['a', 'B', 'c', 'D']
  )

returns

  [1 => 'B', 3 => 'D']

and not

  ['B', 'D']

like every other `filter(...)` i've used...

EDIT to be fair, i see the rationale – having the index of the filtered element is useful sometimes, and requires some contortions with the usual impl of filter. it's quite neat, because you get both the index and the elements! but it's just... surprising as the default, and PHP's conflation of maps and lists obscures it – all the docs say is "array keys are preserved", which makes sense in retrospect, but doesn't really jump out for something with this much impact