PHP +运算符不能一直工作

Question

$var = compact('var1', 'var2') + (object)array(
  'var3' => 'asdf',
  'var4' => 'asdf'
);

不起作用，但为什么它有时会起作用？喜欢

$var = array('var1' => 'asdf', 'var2' => 'asdf') + array('var3' => 'asdf', 'var4' => 'asdf');