If you are using PHP 5.3 on Debian or Ubuntu with php5-gd package installed you will find that you don't have access to the function imageantialias().
Expect to see an error like this in your Apache log file:
PHP Fatal error:  Call to undefined function imageantialias() in /var/www/graphics.php on line 100
This is because the php5-gd package in Debian is not the same as the precompiled binary php GD library found in other distributions. The reason being that it is Debian/Ubuntu policy not to include precompiled libraries for security reasons.
You can verify that the function is missing, along with a few others like imagerotate(), by typing the following in a shell:
php -r "var_dump( function_exists('imageantialias'));"
Full details of this bug (which has been open for over 3 years) can be found here:
There is a workaround which you should only use if you are more concerned with this functionality than the risks of using a precompiled binary which is not subject to security patches: