First upgrade your software as is.
>> sudo apt-get update
>> sudo apt-get upgrade
Then add the following to your /etc/apt/sources.list file.
>> sudo nano /etc/apt/sources.list
deb http://http.us.debian.org/debian stable all deb http://security.debian.org/ stable/updates main contrib deb http://packages.dotdeb.org/ stable all
Finally
run the update to install PHP 5.2.9.
>> sudo apt-get update
>> sudo apt-get install php5-cli
>> sudo /etc/init.d/apache2 restart