code: public function lock($key)
      {
          $this->init($key);
  
          if (!flock($this->files[$key], LOCK_EX)) { //failed
              error_log('ExclusiveLock::acquire_lock FAILED to acquire lock [' . $key . ']');
              return false;
          } else {
              ftruncate($this->files[$key], 0); // truncate file
              // write something to just help debugging
              fwrite($this->files[$key], 'Locked' . PHP_EOL);
              fflush($this->files[$key]);
  
              $this->owns[$key] = true;
              return true;
          }
      }
docs: Lock resource
  @param $key
  @return bool