$cfg['TempDir'] (/tmp) is not accessible


I have edit this file : vim /etc/phpmyadmin/config.inc.php`

and add

$cfg ['TempDir'] = '/tmp';

But I still have this message: $cfg[‘TempDir’] (/tmp) n’est pas accessible. phpMyAdmin est incapable de mettre en cache les modèles et de ce fait sera lent.

(translated in english : $cfg[‘TempDir’] (/tmp) is not accessible. phpMyAdmin is unable to cache templates and therefore will be slow.)

Did I forget a step?

Thank you for your help.

(On a Ubuntu 20.04)

//EDIT : drwxrwxrwt 13 root root 4096 Apr 20 13:25 tmp/

if /tmp is not part of the openbasedir rule that is eventually used for serving phpmyadmin (depends on the vhost used) then of course php cannot access it.