posix_setuid() is broken on snow leopard. why?!

[hazelfelix]

posix_setuid() is broken for me on snow leopard.
I can't get this code snippet to return anything other than Error when executed in PHP 5.3.1 / Apache 2.2.14 on OSX 10.6.3

// Set our real user ID to root
    $success = posix_setuid(0);
    if (!$success)
    {
        print "Error: Cannot setuid().\n";
        return 1;
    }

I know this same code snippet works on my other machine running PHP 5.3.1 / Apache 2.2.12 on Ubuntu 9.10.

Any ideas? I'm going crazy!