Using a stock phpwiki-1.3.14 setup, on Apache 2.2.4 with both mod_php 5.2.3 and 5.2.4, we are seeing an error that causes httpd to exit with an 'Illegal Instruction' error whenever a login attempt is POSTed.
Tracing with gdb, it reveals that the userExists() function of WikiUserNew.php is looping.
I see a comment at the bottom of that function wondering if the code there will prevent an endless loop in all PHPs. Well, for me, anyway, it seems the answer is no.
I'm attaching phpinfo() output and can attach the backtrace from gdb, which
at least can verify what I'm saying.
Happy to provide other information as needed.