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Here's what you do in Ubuntu
1. Make sure all files are owned by the Apache group and user. In Ubuntu it is the www-data group and user
2. sudo chown -R www-data:www-data /path/to/webserver/www
3. Next enabled all members of the www-data group to read and write files
4. sudo chmod -R g+rw /path/to/webserver/www

The php mkdir() function should now work without returning errors