MapWithKey

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Returns an ImmVector containing the values after an operation has been applied to each key and value in the current Pair

public function mapWithKey<Tu>(
  (function(int, mixed): Tu) $callback,
): ImmVector<Tu>;

Every key and value in the current Pair is affected by a call to mapWithKey(), unlike filterWithKey() where only values that meet a certain criteria are affected.

Parameters

• (function(int, mixed): Tu) $callback - The $allback containing the operation to apply to the current Pair keys and values.

Returns

• ImmVector<Tu> - an ImmVector containing the values after a user-specified operation on the current Pair's keys and values is applied.