Retain

:::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. :::

Ensures that this Map contains only keys/values for which the specified callback returns true when passed the value

public function retain(
  mixed $callback,
): HH\Map;

Parameters

• mixed $callback

Returns

• object