Using XPath to extract XML in PHP - php

Question

I have the following XML:
<root>
<level name="level1">
<!-- More children <level> -->
</level>
<level name="level2">
<!-- Some more children <level> -->
</level>
</root>
How can I extract a <level> directly under <root> so that I can run an XPath query such as $xml->xpath('//some-query') relative to the extracted <level>?

Answer 1

DOMXPath::query's second parameter is the context node. Just pass the DOMNode instance you have previously "found" and your query runs "relative" to that node. E.g.
<?php
$doc = new DOMDocument;
$doc->loadxml( data() );
$xpath = new DOMXPath($doc);
$nset = $xpath->query('/root/level[#name="level1"]');
if ( $nset->length < 1 ) {
die('....no such element');
}
else {
$elLevel = $nset->item(0);
foreach( $xpath->query('c', $elLevel) as $elC) {
echo $elC->nodeValue, "\r\n";
}
}
function data() {
return <<< eox
<root>
<level name="level1">
<c>C1</c>
<a>A</a>
<c>C2</c>
<b>B</b>
<c>C3</c>
</level>
<level name="level2">
<!-- Some more children <level> -->
</level>
</root>
eox;
}
But unless you have to perform multiple separate (possible complex) subsequent queries, this is most likely not necessary
<?php
$doc = new DOMDocument;
$doc->loadxml( data() );
$xpath = new DOMXPath($doc);
foreach( $xpath->query('/root/level[#name="level1"]/c') as $c ) {
echo $c->nodeValue, "\r\n";
}
function data() {
return <<< eox
<root>
<level name="level1">
<c>C1</c>
<a>A</a>
<c>C2</c>
<b>B</b>
<c>C3</c>
</level>
<level name="level2">
<c>Ahh</c>
<a>ouch</a>
<c>no</c>
<b>wrxl</b>
</level>
</root>
eox;
}
has the same output using just one query.

Answer 2

DOMXpath::evaluate() allows you to fetch node lists and scalar values from a DOM.
So you can fetch a value directly using an Xpath expression:
$document = new DOMDocument();
$document->loadXml($xml);
$xpath = new DOMXpath($document);
var_dump(
$xpath->evaluate('string(/root/level[#name="level2"]/#name)')
);
Output:
string(6) "level2"
The Xpath expression
All level element nodes in root:
/root/level
That have a specific name attribute:
/root/level[#name="level2"]
The value you like to fetch (name attribute for validation):
/root/level[#name="level2"]/#name
Cast into a string, if node was found the result will be an empty string:
string(/root/level[#name="level2"]/#name)
Loop over nodes, use them as context
If you need to execute several expression for the node it might be better to fetch it separately and use foreach(). The second argument for DOMXpath::evaluate() is the context node.
foreach ($xpath->evaluate('/root/level[#name="level2"]') as $level) {
var_dump(
$xpath->evaluate('string(#name)', $level)
);
}
Node list length
If you need to handle that no node was found you can check the DOMNodeList::$length property.
$levels = $xpath->evaluate('/root/level[#name="level2"]');
if ($levels->length > 0) {
$level = $levels->item(0);
var_dump(
$xpath->evaluate('string(#name)', $level)
);
} else {
// no level found
}
count() expression
You can validate that here are elements before with a count() expression, too.
var_dump(
$xpath->evaluate('count(/root/level[#name="level2"])')
);
Output:
float(1)
Boolean result
It is possible to make that a condition in Xpath and return the boolean value.
var_dump(
$xpath->evaluate('count(/root/level[#name="level2"]) > 0')
);
Output:
bool(true)

Answer 3

Using querypath for parsing XML/HTML makes this all super easy.
$qp = qp($xml) ;
$levels = $qp->find('root')->eq(0)->find('level') ;
foreach($levels as $level ){
//do whatever you want with it , get its xpath , html, attributes etc.
$level->xpath() ; //
}
Excellent beginner tutorial for Querypath

Answer 4

This should work:
$dom = new DOMDocument;
$dom->loadXML($xml);
$levels = $dom->getElementsByTagName('level');
foreach ($levels as $level) {
$levelname = $level->getAttribute('name');
if ($levelname == 'level1') {
//do stuff
}
}
I personally prefer the DOMNodeList class for parsing XML.