It's funny that we feel PHP doesn't make you more productive, yet most of the web was running PHP apps until a few years ago when better languages gained momentum and we saw new web sites not built in PHP.
The decision to use PHP is not represented in the graphs. What the graphs say is that as you gain more experience with PHP you don't necessarily become more productive. 1 yr PHP experience == 10 year PHP experience.
I think your parent is referring to the fact that the Productivity graphed for PHP is the worst of all the languages, even at t=0 (except Haskell, which is shown as starting out with 0 productivity for a while.)
I also strongly disagreed with that for the same reason your parent comment did. The self-assessment bar is funny, but hte blue (productivity) graph needs to be way higher.
There's no absolute scale, so you can't make comparisons between the graphs. The only conclusions you can make from the PHP scale is 1) that (according to the author) PHP developers think they are a lot more productive than they actually are and 2) experience doesn't increase productivity substantially in PHP.
what makes you think the graphs aren't meant to be read on the same scale? By that reading the PHP productivity line could be 100x higher than the next-best language listed, but with self-assessment being 1000x higher than the next-best language listed. That is not a very sensible reading.
Wordpress, phpBB, Drupal, IPB, Joomla, vBulletin, Facebook, Yahoo!, DeviantArt, etc...
Now PHP making you a better programmer... that's a different matter.