Some version of PHP especially since PHP 5, the warning report doesn't appeared, how to solve it?
To solved it by reconfig the php.ini file by follow the steps below.
Open terminal by Applications → Accessories → Terminal
or Ctrl+Alt-t
To make sure, just backup php.ini file before config it, by type this code in Terminal
sudo cp /etc/php5/apache2/php.ini /etc/php5/apache2/php-bak.ini
Open file php.ini by sudo, type in terminal
sudo gedit /etc/php5/apache2/php.ini
Make sure that lines are configed as follow,
error_reporting = E_ALL & ~E_NOTICE
display_errors = On
display_startup_errors = On
track_errors = On
html_errors = On
Save and Close
Restart the server
sudo /etc/init.d/apache2 restart