Dmitriy Koterov has written JsHttpRequest a light wrapper on top of XHR and friends that features:

• Cross-browser compatibility. Library works with IE5.0+, Mozilla 1.7+, FireFox 1.0+, Opera 7.3+, Safari (”+” means “in specified and newer versions”). By this token, code could work without ActiveX and XMLHttpRequest support (but if these capabilities are available, they are used automatically).
• File uploads from user browser to the server without page reloading.
• Full support of PHP debug features. If server-side script fails on an error (including fatal error, e.g., call of undefined function), client-side script can handle it correctly and show diagnostics. Besides response data, it is passed the output flow of server-side script which contains PHP error messages.
• Multi-dimensional data structures exchange (JSON analog) in client request and server response. At that, are used the standard PHP instruments — multi-dimensional associative arrays (request data available at $_REQUEST, response data may be saved to $_RESULT), as well as JavaScript standard capabilities — nested objects and properties. No XML in the level of API: you work with “native” language variables.
• PHP session support using usual way.
• Optimal data loading method choosing (XMLHttpRequest, Microsoft.XMLHTTP,
• Interface is compatible with XMLHttpRequest.

Downloads

• Client side JsHttpRequest.js
• Server side PHP JsHttpRequest.php

AJAX