Hello. I had this problem too, but it's been awhile, so I don't recall the specifics as to how I fixed this. However, be sure that your php.ini file is in c:\windows (assuming you are using windows) so Apache can find it. And make sure you edit c:\windows\php.ini instead of c:\php\php.ini. Check page 14 in the book and it points this out, if you didn't already do that. I think I may have copied c:\php\php.ini over to c:\windows\php.ini when I first encountered this problem.
I also left my libmysql.dll file in c:\php. I have included c:\php in my PATH enviroment variable.
I wish I could recall where I discovered all this stuff...it's been at least a month since I set this stuff up. I'm on chapter 7 now.
Well, good luck. Hope this helps!
**If anyone knows why Apache looks in c:\windows instead of c:\php for the php.ini file, please let me know. I assume its some sore of variable I can set either in a config file or the path environment variable itself.