XMLHttp Requests in PHP

When Microsoft Internet Explorer 5.0 introduced a rudimentary level of XML support, an ActiveX library called MSXML was also introduced (discussed at length in Chapter 4). One of the objects provided in this library quickly became very popular: XMLHttp. The XMLHttp object was created to enable developers to initiate HTTP requests from anywhere in an application. These requests were intended to return XML, so the XMLHttp object provided an easy way to access this information in the form of an XML document. Since it was an ActiveX control, XMLHttp could be used not only in web pages but also in any Windows-based desktop application; however, its popularity on the Web has far outpaced its popularity for desktop applications.

Picking up on that popularity, Mozilla duplicated the XMLHttp functionality for use in its browsers, such as Firefox. Shortly thereafter, both the Safari (as of version 1.2) and Opera (version 7.6) browsers had duplicated Mozilla's implementation. Today, all four browsers support XMLHttp to some extent. (Safari and Opera still have incomplete implementations, supporting GET and POST but no other request types.)