PHP single instance

# PHP Process class for single instance scripts
# source: http://blog.crazytje.be/single-instance-php-script/

# process class

class Process {
    private $strPIDFile;

    function __construct($pProcessIdFile) {
        $this->strPIDFile = $pProcessIdFile;
        if(file_exists($this->strPIDFile)) {
            if(!is_writable($this->strPIDFile)) {
                throw new Exception('File Not Writable', 101);
            }

            $pid = trim(file_get_contents($this->strPIDFile));
            if(posix_kill($pid, 0)) {
                if($this->is_alive($pid)) {
                    //process is alive
                    throw new Exception('Process Already Running', 100);
                } else {
                    //cleanup
                    unlink($this->strPIDFile);
                }
            }
        } else {
            if(!is_writable(dirname($this->strPIDFile))) {
                throw new Exception('Directory Is Not Writeable', 102);
            }
        }

        $id = getmypid();
        file_put_contents($this->strPIDFile, $id);

    }

    public function __destruct() {
        if(file_exists($this->strPIDFile)) {
            unlink($this->strPIDFile);
        }
    }

    private function is_alive($pId){
        exec('ps '.$pId, $ProcessState);
        return(count($ProcessState) >= 2);
    }
}


# example
try {
    $Process = new Process('/tmp/myphpscript.pid');
} catch(Exception $ex) {
    switch($ex->getCode()) {
        case 100:
            echo 'Script already running...';
            return;
        case 101:
            echo 'File Not Writable';
            return;
        case 102:
            echo 'Folder Not Writable';
            return;
    }
}