The first way is using Andphe's PPA. This method is described here.

1. Uninstall all PHP packages.


2. Add the repo by executing sudo apt-add-repository ppa:andphe/php && sudo apt-get update


3. Open Synaptic. Go to Settings -> Repositories. Select the repository we have just added and click "Edit..." Replace "natty" with "maverick". Click "Ok" then "Close".


4. Click the reload button.


5. Now find the php5 package. Press Ctrl + E. Select the version from the PPA. In the main menu, click Package -> Lock Version.


6. Repeat step 5 for other PHP packages.


7. Apply the changes.

N.B. start with "libapache2-mod-php5", "php5-common", "php5-cli" and then the rest

The second way is attempting to install packages built for Lucid. They can be found here.
Just download the packages, dpkg -i them all, and then pin all of them. I will not go into detail because I think that the first way is better.

The third way is installing from source. This guide seems quite good. This is your fallback route.