code: public function offsetExists($offset)
      {
          $task = function() use ($offset) {
              return shm_has_var($this->id, $this->getKey($offset));
          };
  
          return $this->mutex->lockExecute($task);
      }
docs: (PHP 5 &gt;= 5.0.0)<br/>
  Whether a offset exists
  @link http://php.net/manual/en/arrayaccess.offsetexists.php
  
  @param mixed $offset <p>
  An offset to check for.
  </p>
  
  @return boolean true on success or false on failure.
  </p>
  <p>
  The return value will be casted to boolean if non-boolean was returned.