FirstKey

:::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 the first "key" in the current ConstSet

public function firstKey(): ?arraykey;

Since sets do not have keys, it returns the first value.

This method is interchangeable with firstValue().

Returns

• ?arraykey - The first value in the current ConstSet, or null if the ConstSet is empty.