substr in PHP has a funny side-effect such that:

substr("OK,",3) === false

Which causes errors. Fix is as follows:

private function getString($verb, $args = array())
    {
        try {
            $result = $this->doCommand($verb, $args);
        } catch (SeleniumException $e) {
            return $e;
        }
        if (strlen($result) === 3) return "";
        return substr($result, 3);
    }

Note that you could also fix

http://jira.openqa.org/browse/SRC-425

as well as follows:

private function getString($verb, $args = array())
    {
        $result = $this->doCommand($verb, $args);
        if (strlen($result) === 3) return "";
        return substr($result, 3);
    }

Letting the exception be passed up the chain to be handled. I'd also recommend letting this be public to enable overriding it.