yii\web\UrlManager::setBaseUrl PHP Method

This is mainly used when [[enablePrettyUrl]] is true and [[showScriptName]] is false.

public setBaseUrl ( string $value )
$value	string	the base URL that is used by [[createUrl()]] to prepend to created URLs.

    public function setBaseUrl($value)
    {
        $this->_baseUrl = $value === null ? null : rtrim($value, '/');
    }

UrlManager