class MyImplementation extends \DOMImplementation
{
	function commonfunc()
	{
		echo "implementation";		
	}
}

$implementation = new MyImplementation();
$doc = $implementation->createDocument();
var_dump($doc->implementation === $implementation);
// raises error:
//doc->implementation->commonfunc();

.) the objects created by DOMImplementation->create* do not reference the implementation
.) dom_document_implementation_read() always creates a new DOMImplementation object

ext/dom doesn't guarantee reference equality, new instances are often created from properties that behave more like "magic" __get returning new instances based on underlying data.

For this case though, PHP DOMImplementation doesn't support any other implementation than the one in code. Also don't trust hasFeature on it, because that data is just hardcoded.