I was trying to upgrading php 5.2.4 above php 5.2.6 so I could use json.

# cd /etc/apt/root
# vi sources.list

deb http://packages.dotdeb.org/ oldstable all

apt-get update
apt-get install php5-cli

http://ubuntuforums.org/showthread.php?t=1134021&highlight=apt-get+php+5.3+upgrade