Access denied for user 'www-data'@'localhost - how to deal with that?

Question

I face with a strange problem yesterday. I have server running Debian with installed PHP 4.4.4-8 and mysql 5.5.9. That server serves a several websites. For some reason ran

Answer 1

For absent-minded people, this error may happen when mysql_query() is called after mysqli_connect(), when it should be mysqli_query().