framework/caching/XCache.php
protected function addValue($key, $value, $duration)
    {
        return !xcache_isset($key) ? $this->setValue($key, $value, $duration) : false;
    }
protected function addValue($key, $value, $duration)
    {
        return !xcache_isset($key) ? $this->setValue($key, $value, $duration) : false;
    }
Stores a value identified by a key into cache if the cache does not contain this key.
This is the implementation of the method declared in the parent class.

@param string $key the key identifying the value to be cached
@param mixed $value the value to be cached. Most often it's a string. If you have disabled [[serializer]],
it could be something else.
@param int $duration the number of seconds in which the cached value will expire. 0 means never expire.
@return bool true if the value is successfully stored into cache, false otherwise