At some times, server can not change locale
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I have a problem with a PHP script sometimes triggering weird warnings in the Apache error log :

    bash: warning: setlocale: LC_ALL: cannot change locale (fr_FR.utf8)
    bash: warning: setlocale: LC_ALL: cannot change locale (fr_FR.utf8)
    bash: warning: setlocale: LC_ALL: cannot change locale (fr_FR.utf8)
    bash: warning: setlocale: LC_ALL: cannot change locale (fr_FR.utf8)

Below is the code generating this warning. (it is used to configure gettext).

        $lang='fr_FR.utf8';
        $filename = 'default';
        putenv("LC_ALL=$lang");
        setlocale(LC_ALL, $lang);
        bindtextdomain($filename, './config/locale');
        bind_textdomain_codeset($filename, "UTF-8");
        textdomain($filename);

The weird thing is that the warning does not systematically appear. 
If I restart Apache, it will not appear for some minutes (i.e. serving hundreds of requests without any warning) and then it will progressively start appearing.

Thank you