Use Web Sockets. You can use either PHP Sockets with this or this
I think php Sockets would require you to have a PHP version of 5.3 or above.
If you don't have PHP v5.3 or above you can use Node JS with Socket.io
Long Pooling is also one of the option, but that won't be a good solution considering the number of requests your server would be getting. Also the user's bandwidth usage will go high.
