How can I avoid a warning about the PHP mbstring extension? was closed, and I disagree; this message appears when you try to update a module from update.php interface. And to be more clear, my question is about behavior of Drupal if is not installed that extension. I think that it's related with Drupal (please read the question).
You don't install PHP extensions on Drupal 6; you install them on PHP. That is the reason why the question is off-topic on Drupal Answer. If the question can be asked from who is not using Drupal, it means the question is probably not about Drupal.
For example, some of those questions are not on-topic for Drupal Answers.
- What PHP version is compatible with Drupal 6? (It is about Drupal.)
- What is the equivalent for
hook_nodeapi()in Drupal 7? (It is about Drupal.)
- How can I merge two arrays with PHP? (The same question could be asked from who is using Wordpress too.)
It is directly related to the hosting environment, not the installation of Drupal. Someone flagged it as off topic, and I agreed.