Although it is not as fast or small as binary formats, XML's ease of distribution has made it a very widespread format that is pretty much ubiquitous at the enterprise and public sector level.
Basic use of XML is possible with the SimpleXML extension to PHP, but you can take that much further by using both the tree-based extension and the event-based extension - both offer more flexibility.
XSLT support in PHP is excellent, and the technology allows you to manipulate XML in some very cool ways - we even managed to convert XML directly into SQL.
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