I see the mistake here.

Now the question is, where should be this method defined as it's not a escaping method. It seems to me more like a helper to build your query.

That's is correct, but IMHO it's fine where it is. Even though the function doesn't do any escaping, that is also not indicated by the function name. "list_value" would indicate that whatever is returned by the function is a list as string, and the input is an array of items you want to see in that list.
So, I don't see a reason to move this anywhere else. What do you think? Do you know a better place for it?

No, I don't. I thought that you might have. 
From the point that these functions are quite simple I would go with you and leave them where they are.

oops!!!

I agree with the one element, but until which depth do you think that should I check multidimensionality? e.g. array(1, 2, array(3, 4, array(5, 6))) is that an input case in this context?

It's an invalid input case that can happen. I'm actually not quite sure how we want to handle this. implode by default throws a Notice when this happens.
If we want to handle this differently we'd have to go for a foreach instead of an implode.

In the review that I submitted you will find a 1st depth check for multidimensional array. Although it is a proposal I wrote some unit tests for debugging. 
Also wouldn't be better to sanitize the input in a higher level?

I had a look at the changes and I think you missunderstood me, because it's far too complicated.
This is not about input sanitizing, as that should indeed happen higher up. This is about "gracefully" handling invalid input, where "gracefully" is defined by us.
Like I said, implode will throw a Notice in such a case, which is one definition for "gracefully" if that's fine for us. Alternatively it could be something like:

if (is_array($input) === FALSE)
{
    return '';
}

foreach($input as &$value)
{
    if (is_array($value))
    {
        bla;
    }
    else
    {
        foo;
    }
}
unset($value);

Getting an invalud input is something you should always account for, not matter what level you're at. The only question is how are we accounting for it.