Salathe's answer did help me, but since I used the addChild method of SimpleXMLElement, I sought a solution to make inserting children as a first child more transparent. The solution is to take that DOM based functionality and hide it in a subclass of SimpleXMLElement:
class SimpleXMLElementEx extends SimpleXMLElement
public function insertChildFirst($name, $value, $namespace)
// Convert ourselves to DOM.
$targetDom = dom_import_simplexml($this);
// Check for children
$hasChildren = $targetDom->hasChildNodes();
// Create the new childnode.
$newNode = $this->addChild($name, $value, $namespace);
// Put in the first position.
$newNodeDom = $targetDom->ownerDocument->importNode(dom_import_simplexml($newNode), true);
// Return the new node.
After all, SimpleXML allows to specify which element class to use:
$xml = simplexml_load_file($inputFile, 'SimpleXMLElementEx');
Now you can call insertChildFirst on any element to insert the new child as a first child. The method returns the new element as an SimpleXML element, so it's use is similar to addChild. Of course it would be easily possible to create an insertChild method that allows to specify an exact element to insert the item before, but since I don't need that right now, I decided not to.