I was recently browsing around looking for information about PHP SPL apart from the standard info on the php manual website.

I came across the following website: http://www.phpro.org 

This website has some really quality written articles about php and also about the php spl library.

If you a up and coming php developer, or already consider yourself a guru then still check the articles, tutorials as its definatly worth a good look as theres lots of information still to be learnt..