class aClass {
 public function aa() { /* do something */};
}

class bClass {

 private $_obj;

 public function __get($name) {
  if (array_key_exists($name, $this->_obj))
   return $this->_obj[$name];
 	
  // check class - last exist to nowhere
  if (!class_exists($name))
   return false;
   
  // allocate handler
  return $this->_obj[$name] = new $name($this);
 }

 public function bb() {
  $x = $this->aClass->aa();
 }
}