PHP Fatal Error – yii\base\ErrorException - json

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);
}
}