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

Please be note that if you want to know all of the errors even in your coding

just change the

error_reporting=E_ALL & ~E_NOTICE

to

error_reporting=E_ALL & ~E_DEPRECATED

But please make sure that you have quite large harddisk space, if there are many errors in your programming code, the system would generate many of the errors as well in /var/log/apache2/error.log

Save and Close

Restart the server

sudo /etc/init.d/apache2 restart