For information on compiling PHP, see one of the following links. Be warned it takes about 40 minutes to compile the necessary package.

• http://www.phpmag.ru/2009/09/12/ubuntu-9-04-php-5-gd-2/
• http://cumu.li/2008/5/13/recompiling-php5-with-bundled-support-for-gd-on... (Note that comments #9 and #30 can be particularly helpful, as PHP 5.2.* has newer versions since the date of the original post.)