"Execute PHP" form no longer remembers last submitted value

Issue Summary

In Drupal 6, the "Execute PHP Code" form, when submitted, shows the result of the executed code, and prints the same form right underneath it, with the previous value filled in. This can be handy when debugging and you want to run the same bit of PHP over and over - just slap that "Execute" button again. However, in D7, the form comes back blank.

I took a quick peek at the code to see if I could fix this myself, but from what I can tell, the D7 version of the code is pretty much the same as the D6 version, and I can't see what magick is making this happen in the D6 version.