If you want to enable it for all your php projects:
/etc/php5/apache2/php.ini by opening a terminal and typing the following:
gksudo gedit /etc/php5/apache2/php.ini
search for the line.
display_errors = Off
If you want to enable it for a single website, in your website's
.htaccess file add the following line:
php_flag display_errors on
If you edited your
php.ini file, a restart for apache is required for the settings to take effect, restart it with following line in a terminal.
sudo apache2ctl restart