Try this:

  http://www.php.net/manual/en/messageformatter.geterrormessage.php

You might have something like wrong locale there or something..

formatMessage being a static method, it is not possible to use getErrorMessage!

I tested with a full object code:

$fmt = new MessageFormatter("fr_FR", "with accent ? ?");

$fmt isn't an object, but FALSE


Whit
$fmt = new MessageFormatter("fr_FR", "with accent a e");
$fmt is an object, and everything works fine.

Anyway, I must switch back to PHP 5.2, therefore I will recode a very simple formatter instead of use this class.

Well there's your problem. The method isn't static..

No, formatMessage is a static function.

From the doc: 
static  string  formatMessage  ( string $locale  , string $pattern  , array $args  )

@pby_42-web at yahoo dot fr: I guess your string is encoded in ISO-8859-15.
AFAICT, ICU only accepts strings encoded in UTF-8. Converting your string in UTF-8 and using hexadecimal escape sequences, I got the following result:

$ php -a
Interactive shell

php > echo MessageFormatter::formatMessage("fr_FR", "with accent \xC3\xA9 \xC3\xA0", array());
with accent é à
php >

So, MessageFormatter DOES accept accentuated letters, you just have to be careful about their encoding. Maybe the documentation should be updated to state this more clearly.