open_basedir restriction in effect with remote upload avatar

When we try to upload a remote avatar, we can get the "open_basedir restriction in effect" error, like this:

[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_upload.php on line 923: tempnam(): open_basedir restriction in effect. File() is not within the allowed path(s): ([ROOT]:/var/www/clients/client1/web384/tmp:/var/www/clients/client1/web384/web:/usr/share/php:/tmp)

There are many probmems about this: https://www.google.fr/?gws_rd=ssl#safe=off&q=site:www.phpbb.com+phpbb+tempnam+allowed+path

Indeed, when open_basedir restriction is defined (null by default), we need to specify the temporary directory location (with sys_get_temp_dir()) when we call tempnam().