Using variables as object reference for XML - php

Question

Let's say I have a XML file like this one:
<?xml version="1.0" encoding="utf-8"?>
<test>
<foo>
<bar>Hello, World!</bar>
</foo>
</test>
So, if I use something like this I can echo Hello, World!:
<?php
$xml = simplexml_load_file("myxml.xml");
echo $xml->foo->bar;
?>
But, what if I want to refer to bar with a variable?
<?php
$xml = simplexml_load_file("myxml.xml");
$reference = "foo->bar";
echo $xml->$reference;
?>
That won't work. Any solution?

Answer 1

It's because you are trying to access 3rd level in one variable. PHP couldn't handle -> in variable.
$level1 = 'foo';
$level2 = 'bar';
echo $xml->$level1->$level2;

Answer 2

You can't do that because it will look for a property with the literal foo->bar, and not bar inside foo.
You could do it like this:
$xml = simplexml_load_file("myxml.xml");
$reference = "foo->bar";
$tmp = $xml;
foreach(explode('->', $reference) as $v){
$tmp = $tmp->$v;
}
echo $tmp;
Output:
Hello, World!
This would work even if you don't want to go until the last element. Take a look at the following example.
Test with this XML:
<?xml version="1.0" encoding="utf-8"?>
<test>
<foo>
<bar>Hello, World!</bar>
<something>
<values>
<v1>Some value here (1)</v1>
<v2>Some value here (2)</v2>
<v3>Some value here (3)</v3>
<v4>Some value here (4)</v4>
</values>
</something>
</foo>
</test>
Now, change to $reference = "foo->something->values"; and from echo $tmp; to print_r($tmp);. This will be the output:
SimpleXMLElement Object
(
[v1] => Some value here (1)
[v2] => Some value here (2)
[v3] => Some value here (3)
[v4] => Some value here (4)
)

Answer 3

Use Xpath. It allows you to use expressions to fetch parts of an XML.
$xml = <<<'XML'
<?xml version="1.0" encoding="utf-8"?>
<test>
<foo>
<bar>Hello, World!</bar>
</foo>
</test>
XML;
$xml = new SimpleXMLElement($xml);
$expression = 'foo/bar';
var_dump(
(string)$xml->xpath($expression)[0]
);
Xpath is a really powerful tool. However to use the full potential you will have to use DOMXpath::evaluate(). SimpleXMLElement::xpath() can only return node lists as arrays of SimpleXMLElement objects. DOMXpath:evaluate() can return node lists or scalar values.
$document = new DOMDocument();
$document->loadXml($xml);
$xpath = new DOMXpath($document);
$expression = 'string(foo/bar)';
var_dump(
$xpath->evaluate($expression)
);