code: public function clear($uid = null) {
          if (null === $uid) {
              $this->alloc();
              return shmop_delete($this->memory);
          }
          if (!is_int($uid) && !is_numeric($uid)) {
              throw new Exception\RuntimeException('Segment type key must integer or numeric.');
          }
          $this->alloc();
          return shmop_write($this->memory, str_repeat(' ', $this->blocSize), $uid * $this->blocSize);
      }
docs: Clear datas with $uid key
  @param mixed $uid
  @return void