monline

Googling in the subject shows that you can downgrade PHP packages to Lenny versions without much trouble. However the already documented methods lacked a few things I consider important, thus I write down my own downgrade instructions for future reference.

Let's assume that you already have Squeeze up and running (either a clean install or you've upgraded from Lenny).
First of all make a backup of at least your /etc directory and your package selections:
This always comes handy in case you mess things up.

Create an APT sources file for the lenny distribution eg. in a file named /etc/apt/sources.list.d/lenny.list with the following (or similiar) contents:
Note: I've included the proposed updates and backports sources too in case one might need them.

Now set up APT pinning by creating an APT preferences file: either create or edit /etc/apt/preferences or create a new one at /etc/apt/preferences.d/lenny (note that the latter filename has no extension!).
The contents should look something like this:

In the second paragraph list all packages you want to get from Lenny. This will make sure that these come always from Lenny (except if you put here a package that is not in Lenny, but let's assume you won't ). The first paragraph makes sure that Lenny-specific (ie. available only from Lenny) dependencies of packages in the second paragraph are automatically installed as well.

(As a sidenote: you can put either an asterisk ("*") or a space separated package list in the Package: line of the preferences file. It does not accept wildcards in a package name, ie. php5* will not match any package. )

Update your APT caches:

To get a full list of PHP related packages (for the following commands), go through the output of this:

Now purge (deinstall both packages and their config) the Squeeze PHP packages:

If there were any PHP related files/directories not removed during the purge, take a look at them and remove them manually if necessary. Note that probably you should not keep any PHP related stuff, since there's quite some difference between PHP 5.3 and PHP 5.2, and this is true for everything (libs, config, executables, etc.).

After the cleanup (re)install the packages, but first check if your APT configuration is OK.
For this, run apt-get in download only mode:
This will print the URL for every package and you can check what package comes from what location.

(Note: if you're familiar with apt-cache you can verify the repository priorities with apt-cache policy and the package policies with apt-cache policy packagename.)

If every package comes from the right repository (the one you intend it to come from ), install the packages:

The last step is to go through the PHP config files and apply your customizations (if you've any). Since you've a backup of your old PHP config files (your /etc directory), this should be not too difficult. And of course: don't forget to (re)start you Apache.