Detecting if another copy of a program is running (*NIX specific)

One cute trick, to see if another process is running, is to send it signal 0.  Signal 0 does not actually get sent, but kill will check to see if it is possible to send the signal.  Note that this only works if you have permission to send a signal to that process.

A practical use for this technique is to avoid running multiple copies of the same program.  You save the PID to a file in the usual way...   Then during start-up you check the value of the PID file and see if that process currently exists.

This is not totally fool-proof.  In rare circumstances it is possible for an unrelated program to have the same recycled PID.  But that other program would most likely not accept signals from your program anyway (unless your program is root).  

To make it as reliable as possible, you would want your program to remove it's PID file during shutdown (see register_shutdown_function).  That way, only if your program crashed AND another program happened to use the same PID AND the other program was willing to accept signals from your program, would you get a wrong result.  This would be an exceedingly rare occurrence.  This also assumes that the PID file has not been tampered with (as do all programs that rely on PID files...).  

It's also possible to use 'ps x' to detect this, but using kill is much more efficient.

Here is the core routine:

    $PrevPid = file_get_contents($PathToPidFile);

    if(($PrevPid !== FALSE) && posix_kill(rtrim($PrevPid),0)) {
        echo "Error: Server is already running with PID: $PrevPid\n";
        exit(-99);
    } else {
        echo "Starting Server...";
    }

Hmmm...  if you want total 100% reliability, plus efficiency.  What you could do is to make the initial check using kill.  If it says not running, then you are ready to zoom.  But if kill says already running, then you could use: 

//You can get the $ProgramName from $argv[0]
$Result = shell_exec('ps x | grep "' . $PrevPid . '" | grep "' . $ProgramName . '" | grep -v "grep"');

Assuming that your program has permissions to do this.  If you execute that and get back an empty string, then the other program is an imposter using a recycled PID and you are clear to go.  

-- Erik

Although it appears that posix_kill() is built into all modern versions of PHP, it actually requires the "process" extension.  Otherwise, you get an "undefined function" error when you call it.  You might try this to make your code (more) robust on Linux:
<?php
function send_signal(int $pid, int $sig_num) : bool {
    if ( function_exists("posix_kill") ) return posix_kill($pid, $sig_num);
    exec("/usr/bin/kill -s $sig_num $pid 2>&1", $junk, $return_code);
    return ! $return_code;
}
?>
Yes, it works right when $sig_num = 0.

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.

For those that want to kill everything matching a certain pattern (ala killall in for linux), try something like this.  Note that this is a good idea to do something like this for cross platform compatilibity, instead of executing killall, because killall for other UNIXes does just that, kills EVERYTHING.  :)

function killall($match) {
    if($match=='') return 'no pattern specified';
    $match = escapeshellarg($match);
    exec("ps x|grep $match|grep -v grep|awk '{print $1}'", $output, $ret);
    if($ret) return 'you need ps, grep, and awk installed for this to work';
    while(list(,$t) = each($output)) {
        if(preg_match('/^([0-9]+)/', $t, $r)) {
            system('kill '. $r[1], $k);
            if(!$k) $killed = 1;
        }
    }
    if($killed) {
        return '';
    } else {
        return "$match: no process killed";
    }
}

Warning!

The $sig parameter cannot be one of the PCNTL signals constants if you are running your script from the browser (ie. as an apache process). Strangely it does seem to allow the constants to be used from the command line ("php -r ...") but if your script is running from within apache then calling posix_kill($pid, SIGINT) will return FALSE (and posix_get_last_error() will return 0).

check whether the process is running. windows-linux
<?php
function isRunning($pid) {
        $isRunning = false;
        if(strncasecmp(PHP_OS, "win", 3) == 0) {
            $out = [];
            exec("TASKLIST /FO LIST /FI \"PID eq $pid\"", $out);
            if(count($out) > 1) {
                $isRunning = true;
            }
        }
        elseif(posix_kill(intval($prevPid), 0)) {
            $isRunning = true;
        }
        return $isRunning;
}
?>

Posix_kill is not so reliable (it seems it always deliver the signal - even 0 to a no existing process - and doesn't generate an error).
I found this way to check the existence of a process, using /proc:

if(!file_exists("/proc/$pid/cmdline"

obviously check the permissions .

Keep in mind that you can only send kill signals to processes owned by your UID.

If you are running your program as root, then you can send kill signals to all processes.

Just like the error codes (EPERM, EEXIST, etc), signal numbers are different on different platforms.  EG on macos, SIGCONT is 19; on Linux SIGSTOP is 19.  Big difference.

If you have PCNTL compiled in, you can use the constants like SIGCONT; i trust they're all correct.

If not, look in /usr/include/signal.h; these days its tons of ifdef mumbo jumbo but you can go to /usr/include/bits or /usr/include/sys or something and look for files named sig*.h; one of them will list them for you.

here is a process kill function for windows:

<?php
function win_kill($pid){
    $wmi=new COM("winmgmts:{impersonationLevel=impersonate}!\\\\.\\root\\cimv2");
    $procs=$wmi->ExecQuery("SELECT * FROM Win32_Process WHERE ProcessId='".$pid."'");
    foreach($procs as $proc)
      $proc->Terminate();
}
?>

A little recursive function to kill a process and his childs.
it works fine for me and I don't have find something else to do it.
It's a mix of various scripts I've found.

<?php
    function killProcessAndChilds($pid,$signal) {
        exec("ps -ef| awk '\$3 == '$pid' { print  \$2 }'", $output, $ret);
        if($ret) return 'you need ps, grep, and awk';
        while(list(,$t) = each($output)) {
            if ( $t != $pid ) {
                killProcessAndChilds($t,$signal);
            }
        }
        //echo "killing ".$pid."\n";
        posix_kill($pid, 9);
    }
?>

If you want to test whether processes owned by other users are running, you can use:

<?php
  $running=posix_kill($pid, 0);
  if(posix_get_last_error()==1) /* EPERM */
    $running=true;
?>

If the process is owned by somebody else (and you're not root), you will get an EPERM.
On my system (FreeBSD) this is defined to 1.

You should test what the value of EPERM is on your system.