After using the php version downgrade script to downgrade your PHP version from 5.4 to 5.3, you might experience an issue where installing php plugins leads to the below dependency errors. In my case, I was installing a list of packages as advised on this guide.

Update: A new, simpler solution can be found here: 

http://iblog.ikarius.net/index.php/2013/05/01/unmet-dependencies-after-downgrading-php-5-4-to-5-3/

To fix this, you have to unintall PHP 5.3, reinstall PHP 5.4, install your packages, and re-downgrade:

• sudo apt-get remove php5-common
• mv /etc/apt/preferences.d/php5_3 ~/ # move file that tells apt to install version 5.3
• sudo apt-get install php5-common

You can then install your plugins and re-run the downgrade script and everything should work :)

Thanks to ikarius.net for pointing me in the right direction