what is the difference between foo(array(...)) and foo($x=array(...))?

See this example:

This class extends DOMElement. Every DOMElement must be a part of
any DOMDocuement to be modifable.

class Xmlrize  extends DOMElement
 {
     private static $dom;
 
     public function __construct(/*. string .*/ $name,$value =null) {
         parent::__construct($name,$value,null);
     if(!self::$dom) {
             self::$dom = new DOMDocument('1.0','UTF-8');
         }
         self::$dom->appendChild($this);
     }
 }
 

Used to add array into DOM. If value of the array is not Xmlrize, then I ignore it.

class Attributes extends Xmlrize {
 
     public function __construct(array $attributes)
     {
         parent::__construct('Attributes');
 
         foreach($attributes as $name => $value) {
             $element = new Xmlrize($name);
             if($value instanceof Xmlrize)
                 $element->appendChild($value);
             $this->appendChild($element);
         }
     }
 }
 

You can add any DOMNode to the constructor. In my example
it will be instance of Xmlrize

class Html extends Xmlrize {
 
     public function __construct(DOMNode $obj = null) {
         parent::__construct('html');
         if(!is_null($obj)) {
             if($obj instanceof Attributes)
                 $this->addAttributes($obj);
             else
                 $this->appendChild($obj);
         }
     }
 

This method shows if attribute is or is not instance of Xmlrize.

    protected function addAttributes(Attributes $attributes)
     {
         $remove =array();
         foreach($attributes->childNodes as $attribut) {
            if($attribut->childNodes->length == 1) {
                 $item = $attribut->childNodes->item(0);
             echo $attribut->nodeName.': ';
             echo $item->nodeName.' has value '.$item->nodeValue.'. ';
             echo 'Is it DomElement? ';
             echo ($item instanceof DomElement) ? 'true' : 'false';
             echo '. Is it Xmlrize? ';
             echo ($item instanceof Xmlrize) ? 'true' : 'false';
             echo '.'.PHP_EOL;
            }
         }
         return null;
     }
 }
 

Example of initiating of Attributes by array. See the differences!

/* works as expected */
 $like = new Html(new Attributes(
             $xx =  array('XXX'=> new Xmlrize('foo', 'baar'))
 ));
 
 /* do not work as exptected */
 $like = new Html(new Attributes(
               array('XXX'=> new Xmlrize('foo', 'baar'))
 ));
 ?>
 

The example above returns:

XXX: foo has value baar. Is it DomElement? true. Is it Xmlrize? true.
 XXX: foo has value baar. Is it DomElement? true. Is it Xmlrize? false.
 

Why if I add '$xx =' the element is considered as Xmlrize and if not, it isn't?

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