by Gyro
For some reason one of my client's website stopped showing captcha codes for the mad4joomla component, the server is running nginx with php-fpm instead of Apache.
Looking at the error.log, I got
[error] 21110#0: *100 FastCGI sent in stderr: "PHP message: PHP Fatal error: Call to undefined function imageantialias() in /var/www/clients/client1/web1/web/components/com_mad4joomla/sec/im4.php on line 39"
Time to check the server, that function is supposed to be part of the GD library.
First, I checked if GD was loaded:
# php5 -m | grep -i gd
gd
ok, so GD is there… now on to the function.
# php -r "var_dump( function_exists(‘imageantialias'));"
bool(false)
hmm… 
So, on I went to google, trying to find out how I can get imageantialias() to work (again)… to my surprise I actually stumbled upon a post talking about the exact same issue, while using the same server setup -- Ubuntu with ISP Config. The solution presented was to recompile php, which I was reluctant to do.
After some more searching I came across this amazing little page (http://nossie.addicts.nl/php5-gd.html) which I am going to post (slightly modified) right here, mainly because it looks like that page could be gone tomorrow…
Getting bundled php-gd working on Ubuntu without having to recompile php
Like many other people out there, I ran into the problem that the version of php-gd shipped with Ubuntu (and Debian) is different from the version used by many other distributions.
The version Ubuntu uses misses some functions like imagerotate and imageantialias, which are needed by an increasing number of software projects.
One solution to this problem is recompiling PHP with the bundled version of GD.
It is not particularly hard to do, but there are a reasons not to do it, one of them being that it is not neccesary.
The following steps solved the problem for me.
Install what is needed to do this:
# sudo apt-get install php5-cli php5-gd rpm mc
Check what version of php you are running (php5 -v). In my case it was 5.3.2-1ubuntu4.7
Go to rpmfind.net and search for php-gd-bundled
Download the version that matches your PHP version and architecture, php-gd-bundled-5.3.2-1mdv2010.1.x86_64.rpm in my case.
If you have installed mc (midnight commander) and rpm, you can use mc to open the downloaded .rpm file (start mc, goto the .rpm file and hit enter)
Inside the .rpm file you will see CONTENT.cpio, navigate to that and hit enter. Goto usr/lib64/php/extensions, there you will find gd-bundled.so.
The original php5-gd is installed at /usr/lib/php5/20090626/gd.so (for my installation), backup the original gd.so, and copy the gd-bundled.so to that location and rename to gd.so (you can copy files with F5 in mc).
The new gd.so expects to find libjpeg.so.8, this was not present on my system, but that can be sovled by creating a symlink from the installed libjpeg.so
On my system I found /usr/lib/libjpeg.so.64.0.0
Create the symlink with:
# ln -s /usr/lib/libjpeg.so.62.0.0 /usr/lib/libjpeg.so.8
This is it, restart apache and you should be running with the bundled version of gd.
# php -r "var_dump(function_exists(‘imageantialias'));"
Should return bool(true) this time.
Now, if there is an update of the php5-gd package, your modified version gets overwritten. To prevent this from happening, you can hold the php5-gd package so it will not be updated.
# aptitude hold php5-gd
I hope this will help you as much as it did me!
Enjoy 
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