Question about new features of PHP4!

When I first meet PHP, it still in 3.0 version. just after I bought a book abuot PHP3, for a few weeks, PHP4 is released. for now, I have fimilar with some basic of PHP, and I try to find some useful tutorial about the PHP4, I fail. I have look about the PHP manual (pdf version), I find that, there is many many function is added to PHP4, like, session, corba, shockwave flash, XSLT, new function for OO.... But the manual does only provide limited information. I also heard about Zend, and the xxx engine, but just heard. not really know what's that. Can anyone tell me where can I got some article or tutorial, dicussion the new features in PHP4, online (I am a poor student!) Right, I still any another two question, it may be cross post,sorry first. 1. What is "Referer" in a HTTP header, what does it mean? 2. If I have question about mySQL, where can I raise question? Is there any newsgroup which is specially for mysql? Thanks very much!!