With the release of Joomla 3.3 we will see a new system requirement for the installation and upgrade or pre-existing website in the Joomla 3.x series.
To be able to update to 3.3 your server must meet the minimum requirements of PHP 5.3.10 to allow for the required higher level of password encryption that is being integrated into Joomla.
This means there are a few checks that you have to perform before doing the update!
Since the release of Joomla 3.2.2, there has been the introduction of a new system checker before you can update your Joomla website.
So if you update from Joomla 3.2.2 to Joomla 3.3, the installation process of the upgrade will warn you and stop you from upgrading your Joomla website.
If you're on a lower version of Joomla, this won't happen, or if you're upgrading your website through third party services or server level scripts, these may not check settings either so it is best to check your PHP server settings yourself.
It is easy to check, just login to your administrator area and go to SYSTEM -> SYSTEM INFORMATION and the PHP version should show on the first screen that appears. If your server isn't updated, then you might want to change servers, ask your host to upgrade the PHP version or just not update your website just yet. (but you will have to to gain security updates and new features)
You can see more conversation about this topic on the Joomla Google Community.
Michael Babker from the production leadership team has also written a great blog post about the Joomla PHP minimum requirement changes as well.
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