In order to check the outcome of posix_kill() you can use posix_get_last_error(), which will return 0 (zero) in case of success and a non-zero error code otherwise. Use the number returned by posix_get_last_error() as a parameter to posix_strerror() to get a human-readable error message corresponding to that error code.
Please note that a non-zero code from posix_get_last_error() does NOT mean that the pid doesn't exist; it only means that posix_kill() ran into trouble signalling that process. For example, the pid may exist but the process is owned by a user other than the one you use to run the code, and you're not root; in which case you'll get an error saying you're not allowed to signal that process (operation not permitted).
Accordingly, the code posted by Jille earlier is WRONG. According to the POSIX spec (see errno.h on your system), EPERM means "operation not permitted". This should NOT be taken as indication that the pid doesn't exist, it merely means that posix_kill() couldn't signal that process. If anything, it should be a hint that a process with that pid IS running.
errno.h constant names definitions:
http://www.opengroup.org/onlinepubs/009695399/basedefs/errno.h.html
Unfortunately, PHP does not currently define constants with these names (such as EPERM, ENOENT, ESRCH etc.) A non-complete subset is defined for socket operations (SOCKET_EPERM for example), but it doesn't hold all the possible POSIX error constants; ESRCH for instance is of particular interest for posix_kill(), but SOCKET_ESRCH doesn't exist, because it means "no such process" and doesn't make sense for sockets.
Solutions:
* Have PHP devs define these constants in a future PHP version.
* Look-up errno.h on your system and define your own constants. You can use a script to parse errno.h and either define the constants on the fly or generate, once, the PHP code to define them.
Please be advised, however, that relying on a specific errno.h is not portable. Different systems may have different numeric values for these constants. That's why PHP should be defining the constants at compilation time and the code should be able to rely on the constant names; only the names are portable, not the actual values.