Manipulating XML with PHP

[GxH]

I have an XML file that looks like this\[code\]<library> <book> <title>Title</title> <author>author</author> <price>20</price> <book><library>\[/code\]I want to be able to add a book and be able to change the price.Does anybody have any incite on how to do this?Everywhere were i look keeps referring to "simpleXML" however I can only figure out how to create the original file i cant seem to figure out how to edit.