PayPal\Auth\Oauth\OAuthRequest::set_parameter PHP Method

    public function set_parameter($name, $value, $allow_duplicates = true)
    {
        if ($allow_duplicates && isset($this->parameters[$name])) {
            // We have already added parameter(s) with this name, so add to the list
            if (is_scalar($this->parameters[$name])) {
                // This is the first duplicate, so transform scalar (string)
                // into an array so we can add the duplicates
                $this->parameters[$name] = array($this->parameters[$name]);
            }
            $this->parameters[$name][] = $value;
        } else {
            $this->parameters[$name] = $value;
        }
    }