Best version of PHP to learn
This question has probably been asked many times before but I was wondering what would be the best version of PHP to learn as I will be looking to work with Wordpress and other PHP-based CMSs in the near future. I have enjoyed reading the ASP.Net Wrox book which I picked up from an auction website but am now looking at learning more front-end based languages such as PHP.
However, having found a number of books I am a bit confused as to the version number I should pick up. Many books which are quite a bit cheaper cover version 4 though I wonder if it is worth spending more and getting the latest? The highest version number a book covers I have seen is 5.3.