XML as a substitute of RDBMS

I have read somewhere that there are applications/scripts which use XML format to save records rather then a RDBMS, I am going to develop a very light php application, Is it preferred to use XML instead of a RDBMS to save server/RAM resources.

Come on PHP gurus, share your experiences.

It very much depends what your working on, using XML data for applications that have one task updating the data and many tasks reading it work fine, where XML often falls down is if you try and and have multiple tasks updating it at the same time.

Other than that XML is great however depending on the size of the XML and the speed of your parser it can be very slow. Will it save server processor or RAM, again depends how big the XML is and how long you have to keep it in memory. Considering on the average web server the RDBMS will be loaded anyway you wont actually be saving anything, and with an RDBMS you use the amount of memory for that query at that time rather than a whole chunk of XML you may only be using 1 line from.

Jen