varchar lengths

Hi list,

In Oracle I can...
create table a
(
b varchar2(10 chars)
);
...and then, regardless of character encoding and how much space an
ascii character vs a ö takes, 10 characters will fit there.

If I do say a web-thing in php I have to do horrors like...
if (10 < mb_strlen ($b, '8bit'))
{
// fail to explain to the user why it's too long
}

I could be liberal with the length and instead do a...
create table a
(
b varchar(40)
);
...but I like constraints and this isn't a mysql list so I don't have to
motivate that.

Is there anything I've misunderstood? How does the rest of you deal with
this situation?

Thanks,
Marcus