xml xquery and php

[Manish]

I am new to XML. Initially I was saving all the settings in either PHP
or text files or database tables and use to parse the config variables
from files or query from the database. Now I want to keep all the
settings in the XML file and get the setting values, user datas (as
database is not involved and all the results are stored in XML file
itself) from it.

Referring to http://www.w3schools.com/xquery/xquery_example.asp, it
gives sample xml file and also query. But I don't know how to execute
it.
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"books.xml" :

<?xml version="1.0" encoding="ISO-8859-1"?>

<bookstore>

<book category="COOKI NG">
<title lang="en">Every day Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>

<book category="CHILD REN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>

<book category="WEB">
<title lang="en">XQuer y Kick Start</title>
<author>James McGovern</author>
<author>Per Bothner</author>
<author>Kurt Cagle</author>
<author>James Linn</author>
<author>Vaidyan athan Nagarajan</author>
<year>2003</year>
<price>49.99</price>
</book>

<book category="WEB">
<title lang="en">Learn ing XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>

</bookstore>

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doc("books.xml" )/bookstore/book[price<30]

The XQuery above will extract the following:

<book category="CHILD REN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>

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I have a PHP 5 installed on the computer. I want to know, provided the
XML file, how do I execute the query and get the results. Are there
some API, Packages avaailable.

Please also suggest XML user groups.

Thanks.

Manish