How can i simulate REST post request with json data in browser
If you are using Google Chrome you can use the Postman Extension. It allows you to send pretty much any type of data and see the results.
For firefox you can use restclient plugin. Its also very easy to use, and userfriendly.
Also, if you know curl, you can do it programatically from PHP.
If the requests being made through a store in extjs, you can use Ext.getStore to find the store and create global variable. Then you could manipulate this global variable in the console.
i.e. write following in console
var s = Ext.getStore('myStore')
then after firing you can now manipulate this global in console
s.proxy.extraParams.myChangedValue = 'newValue'
s.load()
The response from the server will be in the network panel in chrome browser or use firebug extension for firefox. You can also add a callback or listener to the load event to console.log() what was parsed by the framework.