PHP Fatal Error – yii\base\ErrorException

Question

I have uploaded my site to fpt and I can't fix that error. Please help me.

The error is:

PHP Fatal Error – yii\base\ErrorException

Call to undefined function yii\web\json_encode()

Error is on the line $hash = md5(json_encode($this->rules));

The code is:

public function init()
    {
        parent::init();

        if (!$this->enablePrettyUrl || empty($this->rules)) {
            return;
        }
        if (is_string($this->cache)) {
            $this->cache = Yii::$app->get($this->cache, false);
        }
        if ($this->cache instanceof Cache) {
            $cacheKey = $this->cacheKey;
            $hash = md5(json_encode($this->rules));
            if (($data = $this->cache->get($cacheKey)) !== false && isset($data[1]) && $data[1] === $hash) {
                $this->rules = $data[0];
            } else {
                $this->rules = $this->buildRules($this->rules);
                $this->cache->set($cacheKey, [$this->rules, $hash]);
            }
        } else {
            $this->rules = $this->buildRules($this->rules);
        }
    }