SimpleXMLElement::__construct

The manual doesn't really explain what the $ns argument (and the accompanying $is_prefix) are for.
What they do is similar to the ->children() method: they set the context of the returned object to that namespace, so that access via ->elementName and ['attributeName'] refer to elements and attributes in that namespace.
In particular, they do *not* change the namespaces which exist on the document.
See this example:
<?php
// This XML contains two elements called <child>
// One is in the namespace http://example.com, with local prefix 'ws'
// The other has no namespace (no prefix, and no default namespace declared)
$xml = '<ws:example xmlns:ws="http://example.com"><child>Not in namespace</child><ws:child>In example namespace</ws:child></ws:example>';
$sx0 = new SimpleXMLElement($xml, 0, false);
$sx1 = new SimpleXMLElement($xml, 0, false, 'http://example.com');
$sx2 = new SimpleXMLElement($xml, 0, false, 'ws', true);
echo "
 Without: {$sx0->child}
 By namespace: {$sx1->child}
 By prefix: {$sx2->child}
";
?>

Output:
 Without: Not in namespace
 By namespace: In example namespace
 By prefix: In example namespace

This class is extendable, but it's too bad that its constructor cannot be overriden (PHP says it's a final method). Thus the class should be wrapped using the delegation principle rather that extended.

You won't be able to load an XML file without root element:
//This will throw an exception
$xml = new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8"?>', null, false);
//Here is the solution
$xml = new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8"?><root></root>', null, false);

Extended SimpleXMLElement:
<?php
class XmlElement extends \SimpleXMLElement
{
 public static function factory(string $root): self
 {
 return new static('<?xml version="1.0" encoding="UTF-8"?><'.$root.'/>', LIBXML_BIGLINES | LIBXML_COMPACT);
 }
 /**
 * @param iterable $attributes An array of element attributes as name/value pairs
 * @return $this
 */
 public function addAttributes(iterable $attributes)
 {
 foreach ($attributes as $name => $value) {
 $this->addAttribute($name, $value);
 }
 return $this;
 }
 /**
 * @param string $name The sub-element name
 * @param string|array|null $valueOrAttributes The sub-element value or an array of attributes
 * @param string|null $namespace The sub-element namespace
 * @return static|null
 */
 public function addChild($name, $valueOrAttributes = null, $namespace = null)
 {
 if (is_array($valueOrAttributes)) {
 $child = parent::addChild($name, null, $namespace);
 foreach ($valueOrAttributes as $name => $value) {
 $child->addAttribute($name, $value);
 }
 } else {
 $child = parent::addChild($name, $valueOrAttributes, $namespace);
 }
 return $child;
 }
 /**
 * @param iterable $childs An array of sub-elements as name/value(or attributes) pairs
 * @return $this
 */
 public function addChilds(iterable $childs)
 {
 foreach ($childs as $name => $value) {
 $this->addChild($name, $value);
 }
 return $this;
 }
}
?>

It's worth noting that the behavior of SimpleXmlElement::__construct is not exactly the same as simplexml_load_string, regarding libxml_use_internal_errors().
In my case,
<?php
libxml_use_internal_errors(true);
new \SimpleXmlElement($data);
?>

was still throwing error. But as soon as I switched to 
<?php
libxml_use_internal_errors(true);
simplexml_load_string($data);
?>

everything worked fine and I stopped getting an error.

Using libxml_use_internal_errors() may suppress errors but Exception still requires decent handling. I used following code snippet.
<?php
libxml_use_internal_errors(true);
try{
 $xmlToObject = new SimpleXMLElement($notSoWellFormedXML);
} catch (Exception $e){
 echo 'Please try again later...';
 exit();
}
?>

As I was filling out a bug report, I realized why (speculation here) the constructor is final: so that functions like simplexml_load_file and simplexml_load_string can work. I imagine the PHP-ized code looks something like
<?php
function simplexml_load_file($filename, $class_name = "SimpleXMLElement", $options = 0, $ns = "", $is_prefix = false) {
 return new $class_name($filename, $options, true, $ns, $is_prefix);
}
?>

If we were to use a different $class_name and change the constructor's definition these functions wouldn't work.
There's no easy, sensible solution that keeps simplexml_load_file and simplexml_load_string.