With apache2 and php5 installed you need to make three changes in the file
sudo gedit /etc/php5/apache2/php.ini
Search for entry post_max_size = 2M where you need to enter a larger number than the size of your database (15M in this case) i.e. 25M, next edit memory_limit and give a larger number than the one given to post_max_size. At last, edit the entry upload_max_filesize with a value smaller than post_max_size.
Starting from bigger values the order should go like this
memory_limit post_max_size upload_max_filesize
(for ex: sudo /etc/init.d/apache2 restart) and you are set.