framework/web/UrlManager.php
public function setBaseUrl($value)
    {
        $this->_baseUrl = $value === null ? null : rtrim(Yii::getAlias($value), '/');
    }
public function setBaseUrl($value)
    {
        $this->_baseUrl = $value === null ? null : rtrim(Yii::getAlias($value), '/');
    }
Sets the base URL that is used by [[createUrl()]] to prepend to created URLs.
This is mainly used when [[enablePrettyUrl]] is `true` and [[showScriptName]] is `false`.
@param string $value the base URL that is used by [[createUrl()]] to prepend to created URLs.