@androhit:
Can anyone plz guide me to build this functionality in php or html
'html' (or even php if you which) is need to make your own login page, the one you upload here : Services => Captive Portal => [YOUR ZONE] => Configuration.
Then, have a look at /usr/local/captiveportal/index.php : the file that uses your upload 'htmlt' file.
And, as you can see in this "/usr/local/captiveportal/index.php", this file will tell you everything : /etc/inc/captiveportal.inc - that's will be be the place where you 'interface' with the "Local user manager".
So, two things : you should definitely know something about php, so you can read how things are done, and you will wind up having the knowledge to be able to change the current system to something that needs YOUR typical needs.