PHP: add text to transparent PNG imageSimple way to produce numbered image with PHP. The script uses PHP GD.
PHP: simple captcha script
In my company, we have a lot of WordPress sites. Some of them require contact form with captcha field. The short way for solving such task is to use a plugin. However, as you may guess, the add-ons are available for everyone, which means that they could be easily explored and probably hacked. That's why I suggested a simple, custom script, which is developed by us.