It took me forever to discover that 
DOMDocument->formatOutput = true 
will only have an effect on documents that are loaded from disk if one also sets
DOMDocument->preserveWhiteSpace = false ....

Hope this saves somebody a headache.

The XML parser converts the text of an XML document into UTF-8, even if you have set the character encoding of the XML, for example as a second parameter of the DOMDocument constructor. After parsing the XML with the load() command all its texts have been converted to UTF-8.

In case you append text nodes with special characters (e. g. Umlaut) to your XML document you should therefore use utf8_encode() with your text to convert it into UTF-8 before you append the text to the document. Otherwise you will get an error message like "output conversion failed due to conv error" at the save() command. See example below:

<?php
// Text to insert into XML below
$txt = "a text with special characters like 'ä', 'ß', 'Ü' etc.";

// Create Instance of DOMDocument
$dom =  new DOMDocument;
// Load XML file
// Was created before with DOMDocument('1.0', 'iso-8859-1')
$dom = $dom->load("file.xml");
// Find the parent node
$parent = $dom->documentElement;
// Create Instance of DomXPath
$xpath = new DomXPath($dom);
// new node will be inserted before this node
$next = $xpath->query("//parentnode/childnode");
// Create the new element
$new_elem = $dom->createElement('new_elem');
// Insert the new element
$parent->insertBefore($new_elem, $next->item(0));
// DOMXML = utf-8! (will be converted to iso-8859-1 only at 'save()')
// prevents error message "output conversion failed due to conv error" at 'save()'
$txt = utf8_encode($txt);
// Create new text node with utf-8 encoded string
$nodetext = $dom->createTextNode("$txt");
// Append text node to new element
$nodetext = $new_elem->appendChild($nodetext);
// save 
$dom->save("file.xml");
?>

Hope this helps someone.

siegparr

I just intend to bring my little contribution to the use of DOMDocument::save() function along with PHP serialize() function.

Sometimes you could have to serialize a PHP object before you put it into a XML structure (or other..). And then I suppose you could have to save this XML structure in a file. Of course, you will further need to read this file and unserialize it before to use its content. 

The problem appears when there are some properties described as Protected in the object to be serialized. 

As it’s written in the PHP documentation, serialize function add for those protected properties an asterisk before the name of the property along with a NULL character on each side of the asterisk. 
In this case, the DOMDocument::save function will stop the save operation just before the first NULL character met in the string to be saved, because PHP consider it as a potential risk. 
So, after that, the unserialize operation of the file become impossible.

To solve that, here is a first solution to apply :

For the serialize , before the DOMDocument::save() ::
$data_serial = explode("\x00\x2A\x00", serialize($object));  //take off the 'NULL * NULL' string
$data_serial = implode("\x5C\x30\x2A\x5C\x30", $data_serial);    //and replace with '\0*\0' string 

Before the unserialize :
$data_serial = explode("\x5C\x30\x2A\x5C\x30", $serialized_object);    // take off the '\0*\0' string
$serial_object = implode("\x00\x2A\x00", $data_serial);      // and replace with 'NULL * NULL' string as included by the previous serialize

And a second solution :

For the serialize, before the DOMDocument::save() :
$serialized_object = addslashes(serialize($object)) ;

Before re-using the object in your application :
$object = unserialize(stripslashes($serialized_object)) ; 

Hope this will help some.

When creating a DOMDocument from scratch and saving it, the encoding will be utf-8, although it's declared as iso-8859-1.

When loading an XML-file declared and saved as iso-8859-1, php will keep the correct encoding when it's saved after changes are made. 

Php converts files declared as iso-8859-1 to utf-8 internally. To add text containing special characters, the text must be encoded as utf-8. When the document is saved, special characters are converted to iso-8859-1.

To save an xml created from scratch, use fopen/fwrite and utf8_decode:
   
   $doc = new DOMDocument('1.0', 'iso-8859-1');

//Add som nodes and text with apropriate methods
   
   $f = fopen('xmlfile.xml', 'w+');
   fwrite($f, utf8_decode($doc->saveXML()));
   fclose($f);

I had been hitting my head against the wall when I couldn't get an edited XML file to save due to file permissions, even though I knew that it was OK.

Eventually I realised that for some odd reason, when I went to save the XML file, it would try to write the file to the root directory.

As such, use realpath() to keep the complete system path to the XML file when you load it:

<?php
$myfile = 'myxml.xml';
$myfile = realpath($myfile);
$doc = new DOMDocument('1.0');
$doc->load($myfile);

// Let's just add a couple of elements for good measure
$root = $doc->documentElement;

$title = $doc->createElement('title');
$title = $root->appendChild($title);

$text = $doc->createTextNode('This is a title');
$text = $title->appendChild($text);

$doc->save($myfile);
?>