Just to confirm, definitely a PHP4-related problem. I tried the plug-in on another server that was running PHP 5.2.9 and it installed and loaded up flawlessly.

Fortunately, I do have the option with my hosting provider of changing over to PHP5. I just haven't needed to do so until now.

I'm having the same problem. On further investigation, it looks like the WP-Prowl script is written for PHP5, and the error is occurring on a PHP4 server due to the private/public declarations for the variables and functions.

Of course, the actual ProwlPHP library appears to require PHP5 as well, so those of us stuck on hosting providers that use PHP4 may be out of luck unless somebody wants to take the time to provide a PHP4-compatible version of the library itself.