yii\web\HeaderCollection::offsetExists PHP Метод

This method is required by the SPL interface [[\ArrayAccess]]. It is implicitly called when you use something like isset($collection[$name]).

public offsetExists ( string $name ) : boolean
$name	string	the header name
Результат	boolean	whether the named header exists

    public function offsetExists($name)
    {
        return $this->has($name);
    }

HeaderCollection