PHP-mode-1.5.0's been crashing emacs 23.1. I can trigger this behaviour by entering PHP-mode, then typing
at which point the wrist-watch cursor shows up, and emacs is dead.
I submitted a bug to emacs (5258). See http://lists.gnu.org/archive/html/bug-gnu-emacs/2009-12/msg00613.html for a very long abridged backtrace. It looks like the garbage-collector gets stuck in a loop. I'm not familiar with debugging emacs, so I can't suggest what line of elisp is causing the fail. Previous versions of emacs are not affected.