$agentPrefix = $this->agentId;
$prefixSize = 6;

for ($i = $this->incrementsOffset; $i < $this->incrementsMaxLimit; $i++) {
                $nextIncrementDerProposal = $agentPrefix . sprintf('%0' . $prefixSize . 'd', $i);
                if (!isset($this->busyLogins[$nextIncrementDerProposal])) {
                    $this->loginProposal = $nextIncrementDerProposal;
                    break;
                }
            }