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firstKey

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 Set

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 Set, or null if the Set is empty.

Examples​

This example shows that firstKey returns the first value in the Set. An empty Set will return null as its first key.

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

$s = Set {};
\var_dump($s->firstKey());