Comparing Two SimpleXML Documents

Articles > PHP

As far as I know, there’s no simple way to compare two SimpleXML documents for equality. This fails:

$xml1 = new SimpleXMLElement(file_get_contents('file1.xml'));
$xml2 = new SimpleXMLElement(file_get_contents('file2.xml'));
return $xml1 == $xml2;

And this might too, because whitespace around the tags or swapped attribute order might screw around with the text output:

return $xml1->asXml() == $xml2->asXml();

Technically, even if this was to work (i.e. check that they have the same structure, not the same resource identifier), but the order of the XML documents were different, then it should still fail - XML documents have order.

So I wrote a function which will allow you to check two XML documents and make sure they have the same structure, attributes and children, but in any order (below). Use the optional parameter “$text_strict” to test explicitly for all leading and trailing whitespace.

Usage

$result = xml_is_equal($xml1, $xml2);
if ($result === true) {
   // the XML documents are the same
} else {
   // they are different: print the reason why
   printf(STDERR, "XML documents are different: $result");
}

Code Listing

function xml_is_equal(SimpleXMLElement $xml1, SimpleXMLElement $xml2, $text_strict = false) {
	// compare text content
	if ($text_strict) {
		if ("$xml1" != "$xml2") return "mismatched text content (strict)";
	} else {
		if (trim("$xml1") != trim("$xml2")) return "mismatched text content";
	}

	// check all attributes
	$search1 = array();
	$search2 = array();
	foreach ($xml1->attributes() as $a => $b) {
		$search1[$a] = "$b";		// force string conversion
	}
	foreach ($xml2->attributes() as $a => $b) {
		$search2[$a] = "$b";
	}
	if ($search1 != $search2) return "mismatched attributes";

	// check all namespaces
	$ns1 = array();
	$ns2 = array();
	foreach ($xml1->getNamespaces() as $a => $b) {
		$ns1[$a] = "$b";
	}
	foreach ($xml2->getNamespaces() as $a => $b) {
		$ns2[$a] = "$b";
	}
	if ($ns1 != $ns2) return "mismatched namespaces";

	// get all namespace attributes
	foreach ($ns1 as $ns) {			// don't need to cycle over ns2, since its identical to ns1
		$search1 = array();
		$search2 = array();
		foreach ($xml1->attributes($ns) as $a => $b) {
			$search1[$a] = "$b";
		}
		foreach ($xml2->attributes($ns) as $a => $b) {
			$search2[$a] = "$b";
		}
		if ($search1 != $search2) return "mismatched ns:$ns attributes";
	}

	// get all children
	$search1 = array();
	$search2 = array();
	foreach ($xml1->children() as $b) {
		if (!isset($search1[$b->getName()]))
			$search1[$b->getName()] = array();
		$search1[$b->getName()][] = $b;
	}
	foreach ($xml2->children() as $b) {
		if (!isset($search2[$b->getName()]))
			$search2[$b->getName()] = array();
		$search2[$b->getName()][] = $b;
	}
	// cycle over children
	if (count($search1) != count($search2)) return "mismatched children count";		// xml2 has less or more children names (we don't have to search through xml2's children too)
	foreach ($search1 as $child_name => $children) {
		if (!isset($search2[$child_name])) return "xml2 does not have child $child_name";		// xml2 has none of this child
		if (count($search1[$child_name]) != count($search2[$child_name])) return "mismatched $child_name children count";		// xml2 has less or more children
		foreach ($children as $child) {
			// do any of search2 children match?
			$found_match = false;
			$reasons = array();
			foreach ($search2[$child_name] as $id => $second_child) {
				if (($r = xml_is_equal($child, $second_child)) === true) {
					// found a match: delete second
					$found_match = true;
					unset($search2[$child_name][$id]);
				} else {
					$reasons[] = $r;
				}
			}
			if (!$found_match) return "xml2 does not have specific $child_name child: " . implode("; ", $reasons);
		}
	}

	// finally, cycle over namespaced children
	foreach ($ns1 as $ns) {			// don't need to cycle over ns2, since its identical to ns1
		// get all children
		$search1 = array();
		$search2 = array();
		foreach ($xml1->children() as $b) {
			if (!isset($search1[$b->getName()]))
				$search1[$b->getName()] = array();
			$search1[$b->getName()][] = $b;
		}
		foreach ($xml2->children() as $b) {
			if (!isset($search2[$b->getName()]))
				$search2[$b->getName()] = array();
			$search2[$b->getName()][] = $b;
		}
		// cycle over children
		if (count($search1) != count($search2)) return "mismatched ns:$ns children count";		// xml2 has less or more children names (we don't have to search through xml2's children too)
		foreach ($search1 as $child_name => $children) {
			if (!isset($search2[$child_name])) return "xml2 does not have ns:$ns child $child_name";		// xml2 has none of this child
			if (count($search1[$child_name]) != count($search2[$child_name])) return "mismatched ns:$ns $child_name children count";		// xml2 has less or more children
			foreach ($children as $child) {
				// do any of search2 children match?
				$found_match = false;
				foreach ($search2[$child_name] as $id => $second_child) {
					if (xml_is_equal($child, $second_child) === true) {
						// found a match: delete second
						$found_match = true;
						unset($search2[$child_name][$id]);
					}
				}
				if (!$found_match) return "xml2 does not have specific ns:$ns $child_name child";
			}
		}
	}

	// if we've got through all of THIS, then we can say that xml1 has the same attributes and children as xml2.
	return true;
}

License: MIT