If you are running an old Ubuntu server and you don’t feel like updating the entire server just because your want to install php5-gd please following the directions below:

Command :

sudo apt-get install php5-gd 

Error:

Reading state information... Done
Reading extended state information
Initializing package states... Done
The following packages are BROKEN:
  php5-gd
0 packages upgraded, 1 newly installed, 0 to remove and 4 not upgraded.
Need to get 0B/37.2kB of archives. After unpacking 193kB will be used.
The following packages have unmet dependencies:
  php5-gd: Depends: libt1-5 (>= 5.1.0) which is a virtual package.
Unable to resolve dependencies!  Giving up...
The following packages are BROKEN:
  php5-gd
0 packages upgraded, 1 newly installed, 0 to remove and 4 not upgraded.

FIX

Add this to your /etc/apt/sources.list:
deb http://ubuntu.mirror.cambrium.nl/ubuntu/ hardy main 

sudo apt-get update

sudo apt-get install php5-gd 

php -m (to verify that php5-gd i installed)

service apache2 restart

Reference:
http://packages.ubuntu.com/hardy/amd64/libt1-5/download