src/ElasticSearch/Client.php
public function setIndex($index) {
        if (is_array($index))
            $index = implode(",", array_filter($index));
        $this->index = $index;
        $this->transport->setIndex($index);
        return $this;
    }
public function setIndex($index) {
        if (is_array($index))
            $index = implode(",", array_filter($index));
        $this->index = $index;
        $this->transport->setIndex($index);
        return $this;
    }
Change what index to go against
@return \ElasticSearch\Client
@param mixed $index