I've just finished reading Chris Shiflett's Essential PHP Security, and I have to say that it's a great book. It's very small'"weighing in at only 109 pages (including the appendices and index)'"but I think Chris feels this is its main draw.

Now, for some fun, I used the Rednoize MD5 database mentioned in Chapter 3 of Essential PHP Security to create a little AJAX application to create MD5 hashes of strings, as well as check for the existence of a hash in the MD5 database.

I definitely agree with him on Chris' book - it's a must have for anyone looking to seriously develop PHP applications. And as far as the MD5 script...well, that scares me a little.