JavaScript and PHP are two completely different things, one cannot replace the other. I suggest that you start with a complete package that is already coded in PHP and JavaScript, such as vBulletin.
After trying my hand on Joomla, WP ,Magento and other best PHP scripts through easyphp and the new ampps, I'm now looking to try my hand on a javascript framework which will help me to build a web applications so out of the following two which one should I choose 1) ExtJS or 2) Dojo?
JavaScript and PHP are two completely different things, one cannot replace the other. I suggest that you start with a complete package that is already coded in PHP and JavaScript, such as vBulletin.