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 Vector

public function firstKey(): ?int;

Returns

• ?int - The first key (an integer) in the current Vector, or null if the Vector is empty.

Examples

The following example gets the first key from Vector. An empty Vector will return null as its first key.

$v = Vector {'red', 'green', 'blue', 'yellow'};
\var_dump($v->firstKey());

$v = Vector {};
\var_dump($v->firstKey());