private function getNextVariableName()
    {
        $firstChars = self::FIRST_CHARS;
        $firstCharsLength = strlen($firstChars);
        $nonFirstChars = self::NON_FIRST_CHARS;
        $nonFirstCharsLength = strlen($nonFirstChars);

        while (true) {
            $name = '';
            $i = $this->variableCount;

            if ('' === $name) {
                $name .= $firstChars[$i % $firstCharsLength];
                $i = (int) ($i / $firstCharsLength);
            }

            while ($i > 0) {
                --$i;
                $name .= $nonFirstChars[$i % $nonFirstCharsLength];
                $i = (int) ($i / $nonFirstCharsLength);
            }

            ++$this->variableCount;

            // check that the name is not reserved
            if (in_array($name, $this->reservedVariables, true)) {
                continue;
            }

            return $name;
        }
    }