It’s easy enough to do (although you shouldn’t fix your issues that way, obviously), but I found it difficult to find information about it on the web.
To change your PHP settings in order to avoid these warnings, one quick way is to edit your php.ini file (in /etc/php5/apache2/php.ini) and find the line that says
error_reporting = E_ALL
Possibly it will include other options. A general practical way to avoid the warnings is to extend that line like this:
error_reporting = E_ALL & ~E_NOTICE & ~E_DEPRECATED
Then you’ll have to reload your Apache configuration (apachectl reload or /etc/init.d/apache2 reload)