This is the wrong forum to ask this in, I definitely recommend the PHP forum. However, there are some XML libraries for PHP available (I'm not sure which off the top of my head). If not, you can simply write raw XML files using the fopen() command.
You can probably get more information from the PHP board guys on file reading.
Is this a replacement of my database...
That will completely depend on what the information will be used for. I work in Game Development, and we use alot of XML purely for the readability of the game and level designers. We'd need to know more about the current databases usage, before provided a valuable answer.