Symfony\Component\DependencyInjection\Loader\YamlFileLoader::parseCallable PHP Method

    private function parseCallable($callable, $parameter, $id, $file)
    {
        if (is_string($callable)) {
            if ('' !== $callable && '@' === $callable[0]) {
                throw new InvalidArgumentException(sprintf('The value of the "%s" option for the "%s" service must be the id of the service without the "@" prefix (replace "%s" with "%s").', $parameter, $id, $callable, substr($callable, 1)));
            }

            if (false !== strpos($callable, ':') && false === strpos($callable, '::')) {
                $parts = explode(':', $callable);

                return array($this->resolveServices('@'.$parts[0]), $parts[1]);
            }

            return $callable;
        }

        if (is_array($callable)) {
            if (isset($callable[0]) && isset($callable[1])) {
                return array($this->resolveServices($callable[0]), $callable[1]);
            }

            throw new InvalidArgumentException(sprintf('Parameter "%s" must contain an array with two elements for service "%s" in %s. Check your YAML syntax.', $parameter, $id, $file));
        }

        throw new InvalidArgumentException(sprintf('Parameter "%s" must be a string or an array for service "%s" in %s. Check your YAML syntax.', $parameter, $id, $file));
    }