rest_sanitize_boolean( bool|string|int $value )

function rest_sanitize_boolean( $value ) {
	// String values are translated to `true`; make sure 'false' is false.
	if ( is_string( $value )  ) {
		$value = strtolower( $value );
		if ( in_array( $value, array( 'false', '0' ), true ) ) {
			$value = false;
		}
	}

	// Everything else will map nicely to boolean.
	return (boolean) $value;
}