As I mentioned already, the PHP POSIX functions are really just calls to the standard C POSIX functions, and this has the downside of making them very hard to debug. Two functions alleviate this problem: posix_get_last_error() and posix_strerror(), which grab the last error number and turn that error number into a string respectively. So, if you want to check for an error when calling posix_kill(), you'd use code like this:
In that script, the process ID 293811 is just made up, which is the point: posix_kill() will probably fail because the PID doesn't exist. Or, if it does exist, it might not belong to you, and so also probably will not be killed - unless you're root, of course!