What is the correct way to detect whether string inputs contain HTML or not?

Question

When receiving user input on forms I want to detect whether fields like \"username\" or \"address\" does not contain markup that has a special meaning in XML (RSS feeds) or

Answer 1

HTML Purifier does a good job and is very easy to implement. You could also use a Zend Framework filter like Zend_Filter_StripTags.

HTML Purifier doesn't just fix HTML.