PHP Check Process ID

This is something i have wondered for a while and decided to ask about it.

We have the function getmypid() which will return the current scripts process id. Is there some kind of function such as

checkifpidexists() in php? I mean a inbuilt one and not some batch script solution.

And is there a way to change a scripts pid?

Some clarification:

I want to check if a pid exists to see if the script is already running so it dont run again, faux cron job if you will.

The reason i wanted to change the pid is so i can set the script pid to something really high such as 60000 and hard code that value so this script can only run on that pid so only 1 instance of it would run

EDIT----

To help anyone else with this proplem, i have created this class:

class instance {

    private $lock_file = '';
    private $is_running = false;

    public function __construct($id = __FILE__) {

        $id = md5($id);

        $this->lock_file = sys_get_temp_dir() . $id;

        if (file_exists($this->lock_file)) {
            $this->is_running = true;
        } else {
            $file = fopen($this->lock_file, 'w');
            fclose($file);
        }
    }

    public function __destruct() {
        if (file_exists($this->lock_file) && !$this->is_running) {
            unlink($this->lock_file);
        }
    }

    public function is_running() {
        return $this->is_running;
    }

}

and you use it like so:

$instance = new instance('abcd'); // the argument is optional as it defaults to __FILE__

if ($instance->is_running()) {
    echo 'file already running';    
} else {
    echo 'file not running';
}

In linux, you would look at /proc.

return file_exists( "/proc/$pid" );

In Windows you could shell_exec() tasklist.exe, and that would find a matching process id:

$processes = explode( "\n", shell_exec( "tasklist.exe" ));
foreach( $processes as $process )
{
     if( strpos( "Image Name", $process ) === 0
       || strpos( "===", $process ) === 0 )
          continue;
     $matches = false;
     preg_match( "/(.*?)\s+(\d+).*$/", $process, $matches );
     $pid = $matches[ 2 ];
}

I believe what you want to do is maintain a PID file. In the daemons I've written, they check a config file, look for an instance of a pid file, get the pid out of the pid file, check to see if /proc/$pid exists, and if not, delete the pid file.

   if( file_exists("/tmp/daemon.pid"))
   {
       $pid = file_get_contents( "/tmp/daemon.pid" );
       if( file_exists( "/proc/$pid" ))
       {
           error_log( "found a running instance, exiting.");
           exit(1);
       }
       else
       {
           error_log( "previous process exited without cleaning pidfile, removing" );
           unlink( "/tmp/daemon.pid" );
       }
   }
   $h = fopen("/tmp/daemon.pid", 'w');
   if( $h ) fwrite( $h, getmypid() );
   fclose( $h );

Process IDs are granted by the OS and one cannot reserve a process id. You would write your daemon to respect the pid file.