WP_REST_Request::get_param( string $key )

	public function get_param( $key ) {
		$order = $this->get_parameter_order();

		foreach ( $order as $type ) {
			// Determine if we have the parameter for this type.
			if ( isset( $this->params[ $type ][ $key ] ) ) {
				return $this->params[ $type ][ $key ];
			}
		}

		return null;
	}