Securimage is an free PHP captcha script used for protecting web forms from spam and abuse.

It can be easily installed and very well documented.

Different than the famous reCAPTCHA, which is web-based, Securimage is a downloadable solution that requires PHP and GD library to work.

Some feature of this captcha solution:

• Show an image in just 3 lines of code
• Validate submitted entries in less than 6 lines of code
• Customizable code length
• Choose the character set
• TTF font support
• Use custom GD fonts when TTF is not available
• Easily add background images
• Multi colored, angled, and transparent text options
• Arched lines through text
• Generates audible CAPTCHA files in wav format
• Use a word list for creating CAPTCHA codes

You can find a demo of the captcha here.

Secure input and data handling is hard when it comes to HTML because of many different types of malicious code (XSS).

HTML Purifier is a well documented, standards-compliant HTML filter library written in PHP. It simply:

• Removes all malicious code (better known as XSS) with an audited, secure yet permissive whitelist.
• Makes sure your documents are standards compliant. 

HTML Purifier requires PHP 5  (PHP 4 versions are not supported any more but can be downloaded). It saves so much time while developing & offers much more expertise than most of the self-coded data-handling libraries as HTML Purifier is concentrated only in this area.

This open source secure data handling solution also has a comparison chart wih other HTML filters.

Some community-written plugins for CMS softwares, WYSIWYG editors can be found in the HTML Purifier website.