private function locateFunctionByName(string $functionName) : ?string
    {
        if (! function_exists($functionName)) {
            return null;
        }

        $reflection         = new ReflectionFunction($functionName);
        $reflectionFileName = $reflection->getFileName();

        if (! is_string($reflectionFileName)) {
            return null;
        }

        return $reflectionFileName;
    }