Many functions within the PHP codebase relied upon a universally working getcwd() [C] call to expand paths and to find out where a script is being executed. In particular, Solaris does not assume that getcwd() is a privilege that should be granted to users in directories that don't have 'r' (read) permission, even if it has 'x' (execute) permissions. [...] Under Linux, getcwd() behaves normally but under Solaris, getcwd() does not work with the --x restrictive permissions.

He does note, happily, that this issue is cleared up with an upgrade to PHP 5.2.5 as soon as possible. He had code snippets included in the post so you can test your installation for the problem too.