Set Alias

I am using Mac and its built-in php.

In main.php, i did this

Yii::setPathOfAlias('bootstrap', dirname(__FILE__).'/../extensions/bootstrap');

but it does not work, so, in an index.php, i wrote:

<?php echo Yii::getPathOfAlias('bootstrap');?>

and the webpage show:

/Library/WebServer/Documents/tpl/protected/config/../extensions/bootstrap

that's wrong!

i think is should be

/Library/WebServer/Documents/tpl/protected/extensions/bootstrap

Please how to write the right "setPathOfAlias"?