src/Viserio/Component/Container/ContainerResolver.php
protected function isFunction($value): bool
    {
        return \is_callable($value) && ($value instanceof Closure || (\is_string($value) && \function_exists($value)));
    }
protected function isFunction($value): bool
    {
        return \is_callable($value) && ($value instanceof Closure || (\is_string($value) && \function_exists($value)));
    }
Check if something is a function.

@param mixed $value

@return bool