Advisory notices and warnings from PHP are turned on by default in the php.ini provided in PHP5.3+
These can be turned off by editing your server php.ini (which is the preferred way to solve this, but not permitted by some shared hosts), by using .htaccess or by simply changing the reporting level in Joomla. Go to Joomla 2.5's Global Configuration/Server/Server Settings/Error Reporting. For most users "Simple" is a good setting.
About php error messages: http://www.ostraining.com/blog/joomla/warnings/?mc_cid=5f721f0d11