If there's any interest, I'd also want to create a small tut for using mariadb (replacement for mysql, really cool).
PHP 5.5 (latest) on Debian Wheezy & Squeeze
Before you begin, please back-up all configuration files of your PHP installation (you do not need to touch /usr/local/vesta configuration, because it runs in it's own defined space). This mini-guide is simple, because we're using a 3rd party repository (I use it for over 2 years now and never have a problem and their builds are always very stable. I never had a crash). Do mind that SOME php modules might not be compiled in this repository and thus aren't compatible for your PHP version.
Legal stuff: I cannot guarantee that this will work for you and am not liable in case of any damage that may have been caused by this tutorial.
1. Add the repository and the key to your system from
http://dotdeb.org/instructions (just follow the instructions). You need to add 2 repositories. One for PHP 5.5 and another for general stuff. This repository always updates their versions very fast after a new PHP version has been released. You may also choose to add a mirror instead, see here:
http://www.dotdeb.org/mirrors/
2. Import the key (dotdeb.gpg) as follows: sudo apt-key dotdeb.gpg (can also be found in the instructions from above link).
3. Run apt-get update (make sure you imported your key and repositories first).
4. Run apt-get upgrade and check if everything is upgraded to PHP 5.5 (like php5-common, libapache2-mod-php5 etc.).
5. Make sure that apache still has mpm-prefork (because of the php module). You can check this with: dpkg --get-selections | grep apache2 (you'll get a list with apache2 packages). Make sure the package apache2-mpm-prefork is in that list. If not, it has been removed by upgrading to PHP 5.5. To re-install, run: apt-get install apache2-mpm-prefork (yes, that's all). You can check that PHP is running by going to a hosted domain and by going to phpmyadmin <example.com>/phpmyadmin.
If the login screen of PHP greets you, you have successfully upgraded to PHP 5.5! If not, you might need to enable the php module from Apache. You can do this by using the command: a2enmod php5
Have fun with your new PHP version! PHP 5.5 (as you'll know) also has the Zend OpCache onboard, which can be configured in the default configuration files (their default location).
Hope this helps :) Have fun!