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.