You cannot do the following:

obj->method()->method2();

Mini example:
class Foo{
 var $obj;
  
 setObj( &$obj ) {
   $this->obj =& $obj;
 }

 &getObj() {
   return $this->obj;
}

 hello() {
   print "hi\n";
 }
}

$a = new Foo;
$b = new Foo;
$b->setObj( $a );

$a->getObj()->hello();
// Doesn't work

In fact, this doesn't work either:
function foob() {
  global $a;
  return $a;
}

foo()->hello();

In addition, anything that hands objects back doesn't work
right.  You can't chain certain things, like new commands.

All in all, the objects are rather inflexable.