Jill Burrows has a detailed look at some of the new features coming with the imminent release of PHP 5.4:

What to Look for in PHP 5.4.0

In particular I find the concept of a build-in server for development very interesting, as it makes the barrier to entry into PHP development even lower (you don't even need to set up Apache or Nginx to get going).

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Updated 2022 : note that the above post was originally published in 2011, but is left here for archival purposes.