how can i build websocket application using codeigniter

Handling realtime events properly is very hard and there is a lot of things to think about.

Vital for any production deployment, should be considered when choosing a real-time web technology including:

• Vertical scalability (and performance in general)
  
• Horizontal scalability via fault-tolerant clustering
  
• Guaranteed delivery of data in presence of failures (e.g. if the connection between a client and a messaging server is broken or if the messaging server serving the client goes down, the client will reconnect to another cluster member and will receive all messages published during the reconnection time)
  
• Security of data including data entitlement
  

Source: http://www.leggetter.co.uk/2013/12/09/ch...stack.html

Most of the stuff in above quote applies for large scale project and services, but anything can grow and you might end up with issues pointed out above.

This website is a good resource for information, what to think about and available services and libraries out there.

I always recommend these days to use services that can take care of all the "issues" for you, so that you can spend all your time on building a better experience and website for your users.

I personally have experience with Pusher Realtime service. It starts as free and goes up to enterprise level. It works nicely with CodeIgniter as well and I have created a library that anyone are welcome to use for any project.