What is the best method to merge two PHP objects?

If your objects only contain fields (no methods), this works:

$obj_merged = (object) array_merge((array) $obj1, (array) $obj2);

This actually also works when objects have methods. (tested with PHP 5.3 and 5.6)

foreach($objectA as $k => $v) $objectB->$k = $v;

You could create another object that dispatches calls to magic methods to the underlying objects. Here's how you'd handle __get, but to get it working fully you'd have to override all the relevant magic methods. You'll probably find syntax errors since I just entered it off the top of my head.

class Compositor {
  private $obj_a;
  private $obj_b;

  public function __construct($obj_a, $obj_b) {
    $this->obj_a = $obj_a;
    $this->obj_b = $obj_b;
  }

  public function __get($attrib_name) {
    if ($this->obj_a->$attrib_name) {
       return $this->obj_a->$attrib_name;
    } else {
       return $this->obj_b->$attrib_name;
    }
  }
}

Good luck.