Validator::assertCallback( callable $callback, string $message = 'failed callback assertion' )

    protected function assertCallback(callable $callback, $message = 'failed callback assertion')
    {
        $failing = [];

        foreach ($this->variables as $variable) {
            if ($callback($this->loader->getEnvironmentVariable($variable)) === false) {
                $failing[] = sprintf('%s %s', $variable, $message);
            }
        }

        if (count($failing) > 0) {
            throw new ValidationException(sprintf(
                'One or more environment variables failed assertions: %s.',
                implode(', ', $failing)
            ));
        }

        return $this;
    }