You'll need cooperation from the server-side script, but it is possible to eliminate most of the transfers using conditional requests. HTTP has a feature which allows the server to only send a response if it hasn't changed. For the PHP part, you can use e.g. this module (or reimplement the conditional logic yourself). For the AJAX part, if you use GET (as POST is non-cacheable), the response will be cached and data will only be transferred if it has changed, else it will come from the local browser cache. There are some caveats, especially if your pages are served with an Expires header

You'll need cooperation from the server-side script, but it is possible to eliminate most of the transfers using conditional requests. HTTP has a feature which allows the server to only send a response if it hasn't changed.
For the PHP part, you can use e.g. [this module][1] (or reimplement the conditional logic yourself).
For the AJAX part, if you use GET (as POST is non-cacheable), the response will be cached and data will only be transferred if it has changed, else it will come from the local browser cache.
There are some caveats, especially if your pages are served with an Expires header ([see here][2]).

[1]: http://alexandre.alapetite.fr/doc-alex/php-http-304/index.en.html#download [2]: http://blog.httpwatch.com/2009/08/07/ajax-caching-two-important-facts/