Description:
------------
`Posix_kill` lacks error checking for undefined variables passed and instead gets typecasted to `0` somehow owing to the internal implementation. As specified in the kill(2) manpage, `If pid equals 0, then sig is sent to every process in the process group of the calling process.` Therefore, `posix_kill` will incorrectly send the signal to all processes in the process group which includes the interpreter itself.

This behavior can be detrimental if a developer mistakingly passes an undefined variable to `posix_kill`. This behavior is reproducible with the script specified below. The script sends SIGHUP to the interpreter and exits prematurely due to lack of error checking for undefined variables in `posix_kill`. Therefore, before printing the last echo statement, `posix_kill` passes SIGHUP to the interpreter itself exiting prematurely. 

Test script:
---------------
<?php
echo "Testing Posix kill\n";
$signal_number=1; //SIGHUP=1
posix_kill($DUMMY, $signal_number); // $DUMMY is undefined
echo "Posix kill can error check for undefined variables!";
?>

Expected result:
----------------
Ideally, PHP interpreter should raise an error for an undefined variable for the above test script instead of just weakly typing the undefined variable to `0`.

Actual result:
--------------
The message displayed would be something like:
```
Testing Posix kill
[2] <PHP Interpreter PID> hangup ./sapi/cli/php -f test.php
```