In this post to his blog, Chris Shiflett talks about some issues surrounding character encoding and the cross-site scripting issues it can open up in your application.

In the post [on Good and Bad PHP Code], he provides a few useful PHP interview questions, including some questions from Yahoo. He explains that good PHP code should be Structured, Consistent, Portable and Secure

In the comments, many additional improvements have been suggested, but there's one that has yet to be mentioned. When using htmlspecialchars() without specifying the character encoding, XSS attacks that use UTF-7 are possible.

Included in the post is an example to illustrate the point as well as a solution, a simple one involving the header() function, to help correct the problem and prevent bad things from happening.

Be sure to check out the comments for more great tips.