Some useful notes :)

p_p_lifecycle

p_p_lifecycle=0 > render url

p_p_lifecycle=1 > action url

p_p_lifecycle=2 > serveresource url

you might be have seen p_p_lifecycle in query string many times BUT might be not knowing exactly what is each value exactly mean

It is important to know what are the possible values for p_plifecycle & how they can be helpful, might be useful in forming friendly urls (of Freindly url routes feature of Liferay)

Liferay Sample plugins

For latest version of Liferay released you will have at

http://svn.liferay.com/repos/public/plugins/trunk/

You will have plugins for other versions tagged appropriately here

http://svn.liferay.com/repos/public/plugins/branches/

Liferay DTDs links

6.1

http://cdn.docs.liferay.com/portal/6.1/definitions/

6.2

http://cdn.docs.liferay.com/portal/6.2/definitions/

Liferay's dev guide

https://github.com/liferay/liferay-docs/tree/6.1.x/devGuide/code/devGuide-sdk

layout default variables

http://www.liferay.com/documentation/liferay-portal/6.1/development/-/ai/lp-6-1-dgen05-available-variables-0